EDIT - Just thought have you installed anything recently which you might think has slowed it down ?
If you have a full XP disk a repair will clean up all system files - here is a good guide -
clicky. Although it is very cautious (I've never had to back up .pst files etc) but it is probably worth backing up you valuable stuff just in case. Just make sure you go to the repair screen NOT the repair console (if you do by mistake type exit to reboot)
Next, uninstall any old or unwanted programs and then defrag your hard drive
Stop any unwanted programs for running at startup either from the program itself or via Start > run and type msconfig.
Untick any from the startup tab that you don't want - there is a description of which program the exe is associated with.
There are many more tweeks you can do but these should give the best results.