Yep it is an advert problem, so the forum is not infected. Just don't click any of the ads.
As for clearing the ads and warning Roman, try clearing your cache + cookies. That should clear and temporary files on the forum and should remove the message.
Just to reiterate. The FORUM IS NOT INFECTED. IT IS ONE OF THE ADVERTS. Therefore don't click any of the ads for now until either the admins remove the ads or the ad provided changes them.