Computer experts - how do I remove this "trojan"?

Some websites will not let me access them because of something called ”‘Troj/Rustok-N’ ” Every time I run spyware like Spybot and my Anti-virus program McAffe, it goes away for an hour or so but then it just comes back! I also get pop up ads.

How do I expel this virus/trojan/spyware from my computer?