Dohoho
"McAfee pushed out a virus definition update, 5958, at 14:00 PDT that causes false positive identification of the critical Windows system file svchost.exe. Machines running Windows XP Service Pack 3 using the 5958 definitions will delete the file..."
http://arstechnica.com/business/news/2010/04/broken-mcafee-dat-update-cripples-windows-workstations.ars