I've downloaded and tried three anti-viruses; Avira, Panda and now I'm using Avast and the virus is still here. I just can't stand the fact that I can't navigate anymore without having these annoying spam sites popping-up all the time. I don't even know or at least I'm not sure this is a virus anymore. I just want to know how can I stop these windows that keep popping up in my computer?

these are not pop-ups btw. I have my pop-up blocker program on. These are sites that pop up either with a 'warning: your computer might be infected!' window or on a window promoting an antivirus or a game

Its probably spyware.

Get some anti-spyware programs instead.

I use spy-bot, spyware doctor,
and some other tools.
"HijackThis" among others.

Run them in safe mode as well.

google the problem. if you're having the prob, chances are you're not alone.