Microsoft has taken the unusual step of releasing an unscheduled fix for a problem that caused some PC’s upgraded to Service Pack 2 (SP2) to crash in dramatic "blue screen" fashion.

The problem, which is said to affect all SP2-patched versions of Windows Server 2003, Windows XP Home/Professional, and Windows XP Tablet PC Edition 2005, surfaced a couple of months ago when users started reporting crashes accompanied with the stark message , "Stop 0x05 (INVALID_PROCESS_ATTACH_ATTEMPT)".

Windows XP users will have had the bug quietly fixed in recent days as part of Microsoft’s automatic updates, though users who have disabled this function will still need to download new software. Windows Server 2003 users will have to wait until an unspecified point in the future, however. "There is no hotfix or update currently available to resolve this problem in Windows Server 2003,"said a statement on the website.

Microsoft has been less than forthcoming about the precise origins of the problem, but it appear to relate to the Http.sys system file clashing with TDI drivers loaded by unnamed third-party firewall programs. Although not a straight security issue as such, the problem would appear to reside in that element of the software.

It is not clear why some users might encounter the problem and others would not, as the blue screen crashes have not affected everyone. One possibility is that the issue affects only users of certain firewall programs but that Microsoft is not sure that patching only this would be sufficient.

The full list of Windows software affected by the issue is available on Microsoft’s website. The Windows XP fix can be downloaded from this location.