If you run phpBB on any of your sites (as I do) there is an exploit worm using Google to find and deface sites with a vulnerability on it. The exploit is using a bug in PHP itself (versions prior to 4.3.10.), not phpBB, but phpBB is vulnerable due to the PHP issue.
Go here for a quick fix.
Santy Worm Defaces Web Forums
F-Secure Virus Descriptions : Santy
Codewolf.com is safe.