Oh, god. It's those friends again. I know what it is. See
this thread, just don't post in it.
I also did some further reading about it, and it installs a harmless trojan that just puts stuff in your registry, and you have to buy their software to remove the "viruses" that it finds, which are the registry entries that you can just remove yourself.
I still wouldn't bother with any of it though. The best thing to do when you see this ad is open up the task manager and close your browser. Clicking cancel or whatever just brings you there.