Right Click "My Computer"
Goto System Restore TAB
Turn off System Restore. (Unless you want to keep that feature).
System restore is what makes most XP systems slow down at boot up over time, and the more time spent between reboots it'll be longer. It also degrades over time because of more software.
Its a safety system if something goes to shits you can restore to the last bootup, but it takes lots of space, doesnt work all that well, and is a ressource hog.
Its up to you, but I never keep it turned on.