The presence of a hard-to-detect rootkit may have caused Windows XP machines to freeze up after applying a patch from Microsoft last week, according to preliminary analysis of the problem from Microsoft's security team.

Microsoft's users forums filled up with reports of Windows XP users experiencing the dreaded Blue Screen of Death (BSOD) after applying the 13 patches released by Redmond last week. The problem was later linked to one specific update - MS10-015 - a patch for an "important" kernel flaw - and it was discovered that uninstalling this package unfroze affected machines.

The Blue Screen problem affected a minority of machines but was far from isolated, with many reported cases. Subsequent security sleuthing by sysadmin Patrick Barnes revealed that Windows XP machines that hit a brick wall after applying the update may have been infected with the TDSS rootkit.

Microsoft's security team has since confirmed that the malware may explain the Blue Screen issue in many cases, without ruling out other possibilities.

In our continuing investigation into the restart issues related to MS10-015 that a limited number of customers are experiencing, we have determined that malware on the system can cause the behavior. We are not yet ruling out other potential causes at this time and are still investigating.

Microsoft is asking affected users to send memory dumps in order to aid its ongoing investigation. But it acknowledges this is tricky when users who hit the problem are left with unbootable machines.

Redmond's security team suggested on Thursday that users may want to hold off on the potentially troublesome MS010-015 update and apply a workaround for that particular problem instead. Sysadmins following this advice are strongly advised to apply the other 12 patches issued by Microsoft last Tuesday. ®