Microsoft has re-released a three-week-old security update that rendered many Windows XP machines useless.
Security update MS10-015 is 'incompatible' with a malicious rootkit known as Alureon, according to Microsoft.
After installing the patch, infected users found their PCs would no longer reboot properly. Microsoft quickly yanked the update.
The new version has rootkit detection built in. If a machine is found to be infected, the patch will not install and home users will instead be routed to a Microsoft web site with free telephone support numbers.
The company has also released a Fix It detection tool that security teams can use to figure out if their enterprise PCs are compatible with the patch.
The 015 patch fixed elevation-of-privilege vulnerabilities in the Windows kernel dating back 17 years. While all Windows versions were affected, only XP machines experienced problems with the patch.
Users who have already installed the patch will not have to do so again, Microsoft said.