Virus on my Invision Powerboard 2.2 forums

ok so somehow i got this virus on my IPB 2.2 forums

JS:FakeAV-W [trojan]

http://img27.imageshack.us/img27/2435/27414011.jpg

Basically i can be doing anything on the forums i.e. browsing / view new posts etc then my browser goes to another page (mixscan.com and others) and starts saying i got a virus etc etc

its a scam site/webapge that says i should download a file to remove the virus blah blah

http://img5.imageshack.us/img5/7462/virusomg.jpg

How can i find this Virus??

I have done in IPB Admin tools an executables scan and a check for non IPB files etc but it finds nothing. Ive basically run all the security tools and its found nothing.

Shall i download all the files off my FTP and then run AV tool over them?? My host says they scan every night but they have found nothing. Probably because the virus isn’t actually on my site but rather the trigger to go to another page with the virus etc.

Looking at the folder dates on the FTP nothing has a date newer than a few months and this virus has been on my site about 3 days.

Any ideas? please help this is driving me crazy

What’s the url? Are there any special steps to recreate the problem?