I just finished killing yet another bug - this one was that damned thing that blocks your PC and tells you that "The FBI" has "Detected" you doing "one or more of the following" and then lists all manner of internet crime from software piracy right up to kiddie porn; BUT tells you that if you get a PayPal card and pay a fine of $300, "the FBI" is willing to just forget the whole thing.
This fucker is tenacious. If you get it, FIRST re-start in Safe Mode and do a system restore to a point at least 24 hours before you got this pest. After that, you should be able to get back up on the net.
After your system is restored, get online and download and run "Spybot Search and Destroy". It's free to private users - they do ask for a donation, it's a great tool, so pony up if you can - and scan your system and delete that garbage.