How much RAM do you have? It sounds as if your RAM is getting used up and you're swapping too much. It's also possible that you have some infection or problem not handled by the adware/antivirus programs.
If you have 512mb or more of RAM, it might be wise to submit a HijackThis log in the Hijack forums. If you do, be sure to read the pinned post, "How to Submit a Hijack Log" and don't reply to your own post since the helpers look for posts with no replies. Here
is the link to the forum.