Try getting another browser? Something similar happened to me, but once I got Firefox I was able to get to the other sites.
And Iono if it's the same for you, but some antivirus programs won't open even if you download 'em. If this happens, apparently renaming the .exe file works... I had to do that with FF anyway.
EDIT: Although none of this will permanently fix the problem, to my knowledge. If your computer is used for money or anything and you can (somewhat) easily get a new comp like Shippuden suggested, that may be a good option. Only if you're really suspicious, though. I've had a share of srs spyware attacks and ignore 'em. Course, I don't use a credit card...