Security experts have identified a "very bad" ActiveX-based Trojan, dubbed Offensive, that cracks the Windows registry on an infected PC and kills off the operating system.

The Trojan is transferred via email as an apparently innocuous .html web link.

If it is executed, the target web page displays a button saying 'Start' that, if pressed, destroys the system registry.

On an infected machine it is impossible to run any programs and the OS will lock, preventing a system shut-down. Additionally, it prevents subsequent access to the operating system, even in Safe Mode.

"The virus is very bad if you get it on your system because it totally locks you out," Andre Post, senior researcher at Symantec's Antivirus Research Centre, told vnunet.com.

"It takes quite some knowledge to fix the systems after they have become infected".

Symantec advised home computer users who have become infected with Offensive to seek professional advice, as the only way to repair the damage is using regedit from a DOS command line to fix the damaged registry strings.

The antivirus firm said that it was vital to apply the latest patch for Internet Explorer 5.5 to fight the Trojan.

"The only thing here is that Offensive does not spread by itself. It's got to be manually emailed," Post said.

"If it was self replicating, the threat would be much worse."