Great suggestions already in this thread. The top performance upgrade you can give your computer is enough memory. For Win 9x- 128mb min, WinME- 128-256mb min, WinXP- 256mb+. Check your computer docs to see what's the maximum memory it can use. A few more ideas.
Disk Cleaner - Free program that cleans temp files, recycle bin and other things easily, much easier than 'disk cleanup' in Windows.
Fast Defrag Freeware - Memory management program that frees memory gently, nothing forced.
As mentioned before you may have uneeded programs starting up and using memory and resources.
Disabling Unneeded Startup items
Start, run, type msconfig, Startup Tab. Google each item you're unfamiliar with in the 'Startup item' list to see if its a necessary item by the Operating System. Then uncheck the items you don't want or not required by the OS, then restart computer. The best way to adjust programs is thru each program's startup options, but msconfig works also.
If your computer is still slow after trying these, many people say to reformat once yearly to give the computer a fresh start. With XP that's fairly easy if you have the original OS disk and are ready to backup EVERYTHING you want to save (emails, favorites, programs, etc, etc). But another options with XP is :
XP's No-Reformat, Nondestructive Total-Rebuild Option
With the above method I had no luck making a Slipstream XP CD (probably user error - me). Another Slipstream method from a real Tech Guru.
Slipstreaming Windows XP with Service Pack 2