Windows XP entered beta testing about a year ago, in November 2000. Originally codenamed Whistler, Windows XP forms the core of a family of operating systems, which will be available in Personal, Professional, Data Server and Advanced Server versions, not to mention a 64bit version for the Intel Itanium processor.

Windows XP is undoubtedly the most important operating system released by Microsoft as, for the first time, it unites the old NT and Windows 98/Me codebases into a single entity. For the first time in its history Microsoft has a single, unified Windows platform for every level of deployment, from the home user right up to the corporate server farm.

Although a single codebase had been on Microsoft's wish list for many moons, Windows XP wasn't originally going to be the OS to achieve that goal. As recently as early 2000, Microsoft had two separate OS development projects on the boil: Odyssey and Neptune. Neptune was to be the consumer-based OS, based on Windows NT, while Odyssey was to be Microsoft's next-stage business OS. But by the end of the year both projects had been canned, only to be reborn as a single OS project, Whistler.

Put crudely, Windows XP represents a shotgun wedding between Windows 2000Professional and Windows Me. It gains the rock solid attributes of the former plus the usability, wide driver support and consumer-friendly apps found in the latter. The best of both worlds, in other words.

Two versions

Although there's only one Windows codebase from now on, the so-called Windows Engine, two versions of Windows XP will be available: one for the consumer market, called Home Edition, and one for the business user, the Professional Edition. The latter is a superset of the former - any feature in the Home Edition will also be found in the Professional Edition.

Compared to Windows 2000, Windows XP Home Edition will offer some significant improvements, including a new user interface featuring context-sensitive, task-oriented web views, enhanced support for digital media (including Windows Movie Maker and Media Player for XP), DirectX 8.1 multimedia libraries for gaming, improved software and hardware compatibility, simplified security and fast user switching. Of the two sorts of user Windows XP is aimed at, it appears that the consumer has the most to gain.

The Professional Edition includes features that are deemed unsuitable or too complex for the typical home user. The most obvious difference is security, which is vastly simplified in Home Edition. Each interactive user in XP Home Edition is assumed to be a member of the Owner's local group, which is the Windows XP equivalent of the Windows 2000 Administrator account: this means that anyone who logs on to a Home Edition machine has full control.

Likewise, the Backup Operators, Power Users, and Replicator groups from Windows 2000/XP Pro are missing from Home Edition, and a new group, called Restricted Users, is added. Hidden administrative shares (C$, etc) are also unavailable in Home Edition.

Also omitted are features such as Remote Desktop (which allows a help desk to remotely connect to a client desktop for troubleshooting purposes) and SMP support: Windows XP Pro supports up to two CPUs, while Home Edition supports only one.

Backup and Automated System Recovery have also been cut from the Home Edition - users will have to rely on System Restore to get them out of trouble. There's no fax or personal web server either.

Finally, business users gain support for large LAN-based services such as Active Directory, IntelliMirror and new C2 security features.

XP: a new UI

The most obvious change in XP is its user interface, previously codenamed Luna. Apparently, Microsoft had planned separate user interfaces for the Home and Professional Editions but everybody preferred the former, so the Pro version was scrapped.

The new UI, though based on the Explorer shell, is more extensible and easier to upgrade later. It also allows the user to apply various visual styles, each giving a unique look and feel, while retaining the user's familiarity with the old UI.

The first thing you'll notice about the UI is how bright and colourful it is - Microsoft has taken onboard the fact that everyone now has at least 24bit colour graphics cards and XP takes full advantage of the subtleties such a gamut of colour can provide.

This is further enhanced by a set of brand new icons, which are beautifully designed. Three colour schemes are available - the default blue, a classy-looking silver and a rather dreary olive green. And if all this is scary, you can revert to a classic UI that roughly emulates the Windows 2000 UI. But it's not to be recommended.

Under the hood Explorer has been revamped to simplify OS tasks. Windows 98 made a start, with its web pane views in each Explorer window. In Windows XP, these panes have been substantially revised and made context sensitive - they now list tasks relevant to the current folder. So, a music folder will display tasks for playing music and ordering music online, while clicking on an individual icon reveals choices specific to that file type.

This feature alone makes the new OS significantly easier to use than itspredecessors.

You've always been able to sort files displayed in an Explorer window, e.g. by size or date, but a new category, groups, has been added. For example, you can select group files by name in alphabetical order. You can select by size, and see the files grouped by size categories such as Tiny, Small, Medium and Large.

You can select modification date and see the files grouped by time periods such as Today, Two weeks ago, and Last month, to name but a few. If you open a Music folder, you could group MP3s by Artist or Year, and when you use My Pictures, you can view all your pictures in the folder in a slide show. You no longer need a separate application to view your pictures quickly.

Big changes have been made to the Start menu, which has now absorbed some of the duties previously undertaken by the desktop. It's been turned into a two-pane design that provides quick access to your most important documents, applications and other OS features. This new format uses a task-oriented design to make the Start menu more efficient and user friendly.

In fact, many items you're used to finding on the desktop are now withineasy reach on the Start menu. Now you don't have to worry about open windows obscuring things like My Computer or My Network Places, as they're always available on the Start menu.

Similarly, the Taskbar and tray notification area have been significantly updated as well. Under XP, multiple Taskbar buttons are organised into single logical groups. If you have four Internet Explorer windows open, there will only be one, not four Taskbar IE buttons. When you click on that button, a list will extend up from the button, allowing you to choose which window you'd like.

In the tray, a surfeit of icons reduced the length of the Taskbar; now, under XP, unused tray icons are hidden.

Other major changes

Although available separately for download, Windows XP includes the latestversion of Internet Explorer, version 6. IE6, like its predecessors, is tightly integrated with the OS and naturally adopts the new look and feel of XP.

There isn't a tremendously long list of new features in IE6. At the front of the queue is a new Media Bar, a feature seemingly lifted from MSN Explorer. The integrated Media Bar lets you play back audio and video without opening a separate application using Windows Media technologies - a convenient time saver.

What else is new? An automatic image-resizing feature causes oversized images to shrink to fit the browser window so that you can view it without scrolling. A small floating button lets you toggle such images between shrunken and normal views.

An optional Image toolbar can also be enabled: when you move your mouse over images, the toolbar appears next to the mouse, giving you access to one-click save, print, and email access; it also lets you open the My Pictures folder.

However, one fairly important feature is missing - Java. As a result of an out of court settlement with Sun back in February, Microsoft has decided not to offer the Java Virtual Machine as standard with Internet Explorer 6. If you want it (and many will), you'll be able to download it from the Microsoft website. Sun has an updated version of Java for XP in the pipeline.

Standard support is improved - IE6 fully supports the Document Object Model (DOM) Level 1, SMIL 2 and Cascading Style Sheets (CSS) Level 1. It has also adopted the P3P (Platform for Privacy Preferences) privacy draft recommendations, and IE6 users will be able to prevent access from third-party cookies and other unwanted intrusion using accessible tools.

Another web tool to get a major revamp is Windows Messenger, which replaces MSN Messenger. This was a basic online chat tool that additionally allowed file transfer and latterly Voice over IP.

At first glance, the new Messenger closely resembles the old, but it now incorporates much of the functionality of NetMeeting and so adds two-way audio and videoconferencing, plus application sharing and whiteboarding. It also integrates with the Remote Assistance help facility.

Windows Messenger relies on the newly expanded Microsoft Passport service.

Windows XP now lets you integrate your Windows logon with Passport andWindows Messenger so that you're automatically logged on to Passport-compatible websites when you connect to Windows.

Windows Messenger is actually a key component of Microsoft's upcoming .Nettechnology. Microsoft is reported to be adding a range of .Net related features to Messenger, letting it act as a focal point for services such as MSN Alerts and MSN Calendar.