Many times a PC's 'slow down' is caused by programs running in the background that, that you dont even know about. You can disable these on start up buy changing your MSCONFIG. Its very simple to do and you can disable them from running and tying up system resources. This will also make your PC faster on start up since it doesnt have as many programs to start up.
Heres is a site to get you started if you want. Just remember one rule - when in doubt, leave it on.
http://www.netsquirrel.com/msconfig/msconfig_xp.html