If you've been using computers for a while, you're probably familiar with their flexibility. A modern PC can act as a games system, business tool, education assistant, electronic easel, internet terminal and home entertainment centre.

But did you realise that it's possible to persuade your PC to assume an entirely new personality, to masquerade as a different make and model altogether?

This is the magic of emulation: a show where PCs play-act at being any computer platform they please.

If you use a PC at work but would prefer an Apple Mac at home, then a PC emulator for the latter can bridge the gap to give you the best of both worlds.

Alternatively, if you're nostalgic for computers of a bygone era - the Sinclair Spectrum or Commodore 64 - then relive the good old days by downloading the appropriate emulator.

Should the idea tickle your fancy, keep reading. We'll give you a quick summary of what makes an emulator tick, as well as looking at some of the most successful examples.

Emulation education
An emulator is an application designed to precisely mimic the inner workings of another computer system.

This is possible because, regardless of its make and model, a computer is little more than a collection of microchips, interconnected by a network of electronic pathways.

Such components operate in a strictly predictable fashion and chip designers document the minutiae of their manoeuvres in blueprints and technical manuals.

All a programmer needs to do in order to create an emulator, therefore, is take one of these blueprints and write a program that simulates it.

This isn't quite as easy as it sounds but nor is it particularly difficult. Where problems arise is if the emulated computer is a powerful one, since it needs a correspondingly more powerful PC to run the emulation software.

With one or two notable exceptions, emulation software isn't professionally produced. In fact, most of it is a labour of love, written free of charge by wistful programmers looking to recapture the golden age of computing.

For many programmers that golden age was the 1980s and that's why there are so many Sinclair Spectrum, Commodore 64, BBC Micro and Atari VCS emulators around.

There are also a fair share of video arcade cabinet emulators available too, which open up a whole new world of retro gaming opportunities.

Be warned though: while emulation software offers great nostalgia potential, be prepared for some shocks.

While you may have fond memories of such games as KnightLore and Gauntlet, when held up to the cold emulated light of day, most classic games look rather pitiful by today's standards, particularly when viewed on an unforgiving, high-resolution computer monitor.

Pick a platform
It's not just the PC that can turn its processor to emulating the actions of others. Apple owners can run PC programs on their Macs, while people packing handheld computers can carry around any number of games consoles and computer platforms in their pocket.

However, regardless of which emulation station you use, there are problems. Modern computing devices are powerful machines but their emulation abilities are limited by numerous physical constraints.

A PC can't, for example, alter the physical layout of its keyboard to reflect that of a Mac, and no matter how mighty their talent, a programmer cannot conjure up an adapter to connect an archaic Commodore 64 cassette deck to the hindquarters of a modern PC.

Some commercial organisations and individuals with vested interests complain that emulators are of dubious legality, as the machines they mimic are based on copyrighted designs (see below).

Moreover, their argument continues, emulator users regularly commit software piracy, by downloading ROM images of copyright-protected games and programs.

Emulator advocates, in defence, say that computing history needs to be preserved and that obsolete and otherwise unused computer systems can be immortalised in emulators.

Regardless of either stance, the demand for emulators (as well as the ROM images they launch) runs strong. There are hundreds of emulator programs aping dozens of different machines. Here's the Computeractive guide to the best.

MAME
The Multi Arcade Machine Emulator sits among the elite. In fact, it's the master. Not content with mimicking the actions of a single computing platform, the programming team behind MAME has toiled for six years to develop a program that's able to emulate thousands of machines.

Not just run-of-the-mill computers but full-blown arcade machines, the like of which kids (and we'll wager a fair few adults) feel compelled to feed vast quantities of silver coins.

MAME's Windows Explorer-like interface simplifies navigation of a potentially lengthy list of games titles.

Something of a community project in gaming circles, MAME has been adapted to run on various computers, including PCs, Macs and Pocket PCs, and even some Psion handheld computers.

However, in common with most emulators, MAME is essentially useless without software to run on it.

To avoid inciting the wrath of the law, emulator programmers steer clear of including ROM images with their wares and in MAME's case, locating compatible (not to mention legal) snapshots of arcade machine ROM chips is left to the initiative of users.

In brief, MAME is a work of genius. It strives to emulate every arcade machine that's ever been and for the most part, does so with aplomb. The only downside is sourcing those ROM images and for that, you're on your own.
Price: Free
Download from: www.mame.net

CCS64
This program copies the wheels and cogs of a Commodore 64, in all its blue-screened beauty.

It isn't immediately obvious how to activate anything interesting or useful in CCS64 but if nothing else, that's a faithful reflection of the original computer's foibles.

Pressing the F9 key gets things moving, with a menu that allows you to tweak attributes like screen resolution and colour usage. It's here also that game images (of either cassettes or Commodore 1541 disks) can be loaded.

The emulator is available in two flavours: MS-DOS or Windows. It goes without saying that the Windows version is easier to install and use but, in our experience, the MS-DOS edition does a slightly better job of reproducing the sounds and speeds of all those original games.

CCS64's author, Per Hikan Sundell, keeps a selection of popular Commodore 64 games titles available for download on his website but is at pains to point out the legalities of their use. We like CCS64 but it would definitely benefit from a more Windows-like front end.
Price: Shareware, $30 (£18) registration.
Download from: www.computerbrains.com

Z80
More than any other aged computing platform, Sir Clive Sinclair's Spectrum has been attracting emulator makers in droves. A quick tally reveals no fewer than 18 programs designed to emulate the very same unsophisticated Sinclair hardware.

By dint of its simplicity, Gerton Lunter's Z80 has to be our favourite Spectrum emulator. With versions for DOS and Windows, it makes experiencing old 'Speccy' games as straightforward as opening a Word document.

For all the program's inner goodness, however, the author is at risk of alienating users from the off. To be allowed to step back in time, you'll find that before you can register you're forced to play an intensely frustrating game of 'catch me if you can' with an OK button. You might decide not to bother.
Price: Shareware, £15 registration.
Download from: www.worldofspectrum.org

Virtual PC 5 (Mac/Windows)
If you've got a Mac, installing Virtual PC will allow you to install Windows and experience the wider world of PC software, although most games will remain unplayable.

If you buy the PC version, you might be wondering why you've bothered, since you need a PC to run it.

Virtual PC for Windows offers a quick and easy way of installing and using additional operating systems, without having to deal with the hassle and usual complication of hard disk partitioning and so on.

The program made its name in the Apple arena though and, if you've got a sufficiently speedy Mac, Virtual PC does a good job of recreating a Windows PC environment on the Mac desktop.
Price: £175 (Mac edition); £199 (PC edition).
Download from: Computers Unlimited 020 8358 5857
www.connectix.com

PocketVCS
Today, Sony's PlayStation is synonymous with gaming but a generation ago, another name was embedded in players' minds: Atari.

In the 1980s, the firm's VCS console sold in the millions worldwide and spawned the release of thousands of game titles.

If you've got a Pocket PC, you can now carry around with you every last one of these games and play them on the PocketVCS emulator.

It offers near-perfect emulation of the original Atari 2600 VCS console, replete with on-screen switches and knobs. The author has also produced a Coleco console emulator, and both are well worth a play.
Price: Free
Download from: pocketvcs.emuunlim.com

MESS
We've almost run out of space for reviews but if we haven't yet mentioned an emulator for your favourite computer platform from days of yore, fret not - we've a fine MESS for you to get into.

MESS stands for Multi-Emulator Super System and the program is to home computer emulation what MAME is to the arcade-aping arena. Rather than recreate a single computer platform, MESS digs up dozens.

It's an attempt to emulate almost every home computer of the past two decades, from the Amstrad 464 to the ZX Spectrum +3.

In operation, it looks and feels a lot like MAME but, because you'll probably have to find the necessary ROM images for your preferred computer, it's not so immediately useable.

In brief, while there are better Spectrum and Commodore emulators out there, if you really want once more to relive the glory days of the Oric Atmos, Aquarius, BBC Micro, Dragon 32, Jupiter Ace, MSX or Vectrex, then short of someone inventing a time machine, MESS could provide the only opportunity to do so.
Price: Freeware
Download from: www.mess.org

CONCLUSION

So there you have it, five free and one not-so-free ways to make your PC think it's something else and, if you dig around on the internet, you'll find at least a dozen more options.

Some of the emulators take a bit of figuring out but, if you remember the golden age of computing when many of these systems first appeared, you'll remember that head-scratching was all part of the fun. So, enjoy.

LEGAL ISSUES

Emulation is a legal minefield and anyone engaging in the activity risks breaking any number of copyright laws.

Just because numerous authors make various emulator programs available to download free of charge, it doesn't follow that their use is legal.

Even if the manufacturers of the emulated computers and the associated software have long since gone out of business, it's entirely likely that someone, somewhere, will retain rights over the use of the properties.

The law is so grey in this area that it seems few people or organisations are prepared to comment on the existence and use of emulators.

During the compilation of this feature, we contacted a variety of involved parties - software programmers, hardware manufacturers and industry bodies - and few were prepared to respond in print.

The recently revamped version of Amstrad's E-m@iler telephone-cum-internet terminal includes a feature that allows users to download and play old Spectrum games.

Amstrad owns the Sinclair brand and related Spectrum computer technologies, so we were interested in the firm's view on the proliferation of Spectrum emulators and software snapshots. Sadly, the company chose to ignore our requests for comments.

Sony famously spent 18 months and untold millions pursuing an outfit called Bleem through the courts. Bleem produced an emulator that enabled games fans to enjoy PlayStation titles on their PCs.

Sony made it clear from the outset that it wouldn't rest until Bleem was no more and eventually, the Japanese giant was granted its wish. Bleem wound up its affairs in November 2001.

We asked the European Leisure Software Publishers Association's (ELSPA) for its stance. The organisation took the best part of a week to decide that it wasn't able to comment on the legal aspects of the emulation phenomenon.

Pushed for a response, an ELSPA representative would say only that the situation was a legal quagmire and that the association had yet to form a crystallised view.

It's not all muddy news, though. Many copyright-owning authors of classic games are benevolent with their early works. Surf on over to gremlinworld.emuunlim.com, for instance, and you'll find a near-complete back catalogue of games from the Gremlin Graphics stable.

Generously, the holder of the Gremlin Graphics software properties, Andy Davis, has made ROM images available of all the games titles he owns. If you want to be certain of staying on the right side of the law, you'll have to seek out similar legal sources.

EMULATION ONLINE

If the emulator scene interests you, there are a few websites worth a visit. Top of your list should be ClassicGaming.

This site has a good database of emulators, though they are not always the most up-to-date versions. There's also a vast repository of ROM images for a multitude of formats.

Another interesting link is the Java Gaming & Emulation Portal. Here you will find information about and connections to websites that feature Java-based emulators.

If you fancy a quick nostalgia trip, these offerings can provide an emulator in your web browser window.