McAfee corporate customers across the globe using Windows XP experienced massive shutdowns today as an update to McAfee's security suite deleted svhost.exe — a very common Windows process — causing machines to reboot continuously. McAfee has confirmed the problem and removed the faulty virus definition file from the update. Only McAfee's corporate users were affected.
If you or your workplace were affected, Twitter user scratchfury offers this (unverified) tip:
boot to safe mode, rename mcshield.exe, reboot, run Virus Console, pick Tools -> Rollback DAT, name back to mcshield, reboot
We don't have a machine to test it on, so follow his tip at your own risk, and only if you know what you're doing. If you've fixed it already, let's hear how you did it in the comments.