Microsoft will rush out an emergency security patch for Windows users on Thursday.

The company offered few details on why it was releasing the software update, which is rated critical for users of Windows 2000, Windows XP, and Windows Server 2003. A critical flaw is worrisome, however, because it can be exploited by online attackers to seize control of the PC.

The update will be released at 10:00 am, Pacific time, said Microsoft spokesman Christopher Budd in a blog posting published late Wednesday.

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 Web sites.

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

For Microsoft to rush out this type of emergency update, it must consider the bug to be very serious, said Dragos Ruiu, organizer of the CanSecWest hacking conference in an instant message interview.

Ruiu said that presenters at Microsoft's recent Blue Hat internal security conference told him that 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.