PC's running the popular McAfee VirusScan were brought down recently when the anti virus program began attacking core system files. Based on anecdotes the 'attack' appears to be caued when older VirsuScan engines install DAT 5664 which the anti virus giant pushed out within the last 36hours.
Affected systems then start to identify a wide variety of legitimate and frequently crucial system files as malware. According to this article files belonging to Microsoft Internet Explorer, drivers for Compaq computers and even the McAfee-associated file McScript.exe were being identified as a trojan called PWS!hv.aq.
I can only imagine what admins the world over are going through right now. Hopefully will get a lid on this situation ASAP.