I've been battling similar problems though mine is really related to a hog of
an application. I second the suggestion to get the best Virus and Spyware
protection that you can. It is truly amaing how much junk other people can
put on your machine.

Two things that I didn't see mentioned are: A general register scrubbing - I
can't recommend a tool because that's when I go for the complete rebuild.
(2) Some MS Page recommended "Exective Software DiskKeeper". I bought it and
I am glad that I did. It must be 300% faster than diskscan, and 100% faster
than Windows Defrag. It also does a much, much better job of defragging
which will definitely be a performance improvement. It helped me a lot, and
I ran before and after bench marks to prove it.