Have you defragmented the hard drive lately? You can use the windows defragmenter under the System Tools Menu in the Start Menu. I would recommend getting a program like Diskeeper though, thats what I use. It allows you to auto-defragment your hard drive like during your screen-saver or late at night.
Other than that, I would recommend Ad-Aware and cCleaner for cleaning up your computer.
Of course adding more memory to your computer would always speed it up
