From Wikipedia:

> On April 21, 2010, beginning at approximately 14:00 UTC, millions of computers worldwide running Windows XP Service Pack 3 were affected by an erroneous virus definition file update by McAfee, resulting in the removal of a Windows system file (svchost.exe) on those machines, causing machines to lose network access and, in some cases, enter a reboot loop. McAfee rectified this by removing and replacing the faulty DAT file, version 5958, with an emergency DAT file, version 5959 and has posted a fix for the affected machines in their consumer knowledge base.

BBC article from the time: http://news.bbc.co.uk/1/hi/technology/8636985.stm