One other thought. Are you sure the pop-ups are coming as the result of visting certain websites, eg. do you get any problems visiting Mudcat which doesn't have popups for example?

I only ask as I did something incredibly stupid on my laptop a couple of weeks ago - I'd tried to download a zip file, realised when the "do you wish to download and install..." type box came up that it wasn't a zip file but in my haste, clicked OK! The end result was I got plagued with popups when I used IE or Firefox because of the malware I'd managed to install.