Run a Disk CleanUp first...
From XP go to...
Start -> Control Panel -> Performance and Maintenance -> Free Up Space on Your Hard Drive
This will analyse what files are on your hard drive that can be removed - usually temporary files, and old unused programs.
Once you've cleaned up your hard drive, run a defragmentation...
From XP go to...
Start -> Control Panel -> Performance and Maintenance -> Rearrange Programs on Your Hard Drive to make programs run faster
This will do exactly what it says on the tin - make sure you run this overnight, or sometime when the computer will be out of use for a few hours. Also make sure you turn off your screensaver, as I've sometimes seen this interrupt the process, adding another few hours onto the time.