One thing you might wanna try is to slipstream/customize your next XP install CD.
nLite is a program where you can configure exactly what does, or doesn't, get installed when you run the Windows installation. It'll help take out the crap you don't need. Also it can/will integrate the 100's of Windows security updates into the CD itself.
TweakUI is an excellent program, as well as turning things off in msconfig (use with caution!). Those will help speed up your system.