Microsoft issued a rare emergency patch on Thursday to fix a critical bug in its Windows operating system, which could be exploited by online criminals to take over individual and corporate computers.

It was the first time since April 2007 that the software giant has issued a patch outside its schedule of monthly updates.

Microsoft said the critical flaw affected computers running Windows XP, Windows 2000 and Windows Server 2003, with less danger posed to computers running Windows Vista and Server 2008.

"If successfully exploited, an attacker could then install programmes or view, change, or delete data; or create new accounts with full user rights," a Microsoft bulletin said.

The downloadable software was available at http://www.microsoft.com/technet/security/Bulletin/MS08-067.