I had a similar problem a couple of months ago. Our corporate firewall blocks all porn sites, but this stuff got through anyway. I still feel I picked it up on one of the gun forums I visited, but not sure which one (Glock, Sig, AR15,etc.) My IT group used AdAware and Spybot to attempt to eradicate the problem, but were unsuccessful. Hopefully you have greater success. If not, you will probably have to reinstall your OS in order to wipe out the registry entries that it is using to "resurrect" itself, a real pain in the butt. I never download unknown items nor give them permission to install. If I find out who did it, I would love to take legal action against them. I now run a trojan monitor at all times. If anything tries to "download" in the background, it notifies me immediately. Be careful what you "click" on, no matter where your surfing, including forums.
Good luck with your problem. [:(!]