Disable all processes not needed
Disable all services not needed
Tweak all services that can be (from auto to manual or appropriate) - NB check and double check via Black Viper or similar.
(PM me on processes / services if you like)
Set swapfile to a fixed minimum of appropriate size (I'm on Vista, so forget the best settings for XP). No maximum. Let XP manage that as required.
Delete as much carp off the system as possible and ideally get no more than 50% of C: in use. Look in places such as mail deleted folder, mail sent folder and other mail folders for opportunities to delete stuff, especially e-mails with attachments.
Clean out the system restore points, and set a maximum disk space for it, so that 4-6 restores are recycled. Anything more is probably a waste of space.
Un-install everything (which is obsolete, expired trials, never used).
Shift game installations to another drive if possible (best to un-install and re-install)
Zip up any files in mydox not accessed in the past year (deleting the original afterwards of course).
Remove fancy or large imagery from the desktop (convert big images to desktop size and save as smaller format/quality), and reduce the icons on the desktop. Get rid of pop-up taskbars which are rarely used, and add the used icons to the main taskbar (e.g Office start bar).
Ensure recycle bin size is set to a realistic size to free up disk space.
Disable Windows indexing
Disable or tweak scheduled tasks, such as Windows update (and another updates, such as Adobe, java etc), Anti-virus scans, defrag.
If using 3rd party AV and firewall, consider disabling Windows AV / firewall.
That ought to work.