Microsoft is set to rush out an emergency security patch for Windows users today.

Microsoft has offered few details on why it is releasing the software update, which is rated 'critical' for users of Windows 2000, Windows XP and Windows Server 2003. Many security experts are concerned, however, because a critical flaw can be exploited by online attackers to seize control of the PC.

The update will be released at 10am US Pacific Time (6pm tonight for UK users), according to Microsoft spokesman Christopher Budd, in a blog posting published late last night.

The flaw is considered to be a less serious risk for users of the Windows Vista and Server 2008 operating systems, Microsoft said in an advisory on the issue.

For years, Microsoft has released its patches on a predetermined day - the second Tuesday of every month - but it has occasionally released patches ahead of schedule when bugs have been actively exploited by computer criminals.

The last such emergency patch issued by Microsoft was in April 2007, when the company fixed a bug in the way Windows processes .ani animated cursor files. That flaw was publicly known and being exploited in attack code hosted on hundreds of websites.

This latest vulnerability, however, appears to be unknown to the security community.

Dragos Ruiu, organiser of the CanSecWest hacking conference, commented that for Microsoft to rush out this type of emergency update, it must consider the bug to be very serious.

Ruiu said that presenters at Microsoft's recent Blue Hat internal security conference told him they'd discovered some serious Windows bugs using security testing tools and that the update could fix one of these issues. "It might have wide reaching impact, or might be used easily for significant malicious hijinks," he said.

Click here for the best Microsoft voucher codes.