Upgrade it if possible.
Re-format it and run a clean install of your OS, that usually helps. Just minimize clutter and try and get your HDD as empty as possible. The more memory you have the better. Try to monimize system processes.
Alot of people here have 50+ processes running. That's OK if you have a very powerful machine with tons of memory to spare. But for older machines...that will kill performance. I like to keep mine 20 or lower, even on a very fast computer.