I've had this twice in the last two days, and I think we should all look on the bright side. At least it wasn't actually from Microsoft.
Amen. Their recent Critical Update to fix a DAO vulnerability on XP/2k did an excellent job of killing my XP/Pro box (one of the threads in the System process went in to a tight loop using 100% CPU). Had to rollback XP to the initiall install and repatch - only around 60 critical updates
. Microsoft are "aware" of the issue, which affects a "small number of systems" and "may be fixed in SP2".