If it's the same one that's been doing the rounds it's pretty darn nasty -- the Boards.co.uk forum has been having the same trouble.
From memory it's a MySQL injection attack, only windows servers are vulnerable, and the attackers load all sorts of malware onto the website so that anyone who logs on gets infected.
archive.f-secure.com/weblog/archives/00001427.htmlI hope they backed up recently...