disable system restore
download regseeker and do a full registry scan, and remove all green + red entires
(after regseeker) use ntregopt to shrink registry size (sort of like defrags it)
create a new user account and use that instead (mydocuments and desktop will need to be copied across)
update drivers for your hardware
remove all unneeded things from add/remove programs
disable services you don't need (services.msc)
uninstall windowsdefender or any other antispyware programs that constantly run..
if you have xpsp2 and a router, then uninstall antivirus (just don't visit any silly websites)
these three are a bit more drastic
reinstall xp
get more ram
get faster hard drive