All I can think of is to download Ad-Aware to get rid of alot of your possible spyware and cookies.
It is free. It has caught alot of stuff on my computer that a regular cleaning of the registry doesn't catch.
Is your computer up to date on those annoying Windows updates? Sometimes the updates do some good(more often than not they are a pain in the behind) but it might work.
Is your virus software up to date?
I really can't think of anything else to help. Sorry.
I just thought of something, sometimes if you do a system restore, the system will reset itself to what ever point you choose and you might gain some speed.
I hope someone can come by who can help you much more than I can.