You might not have enough memory.
You might have a virus.
You might have a bad driver.
XP might have become corrupt and needs to be reinstalled.
Your hard drive might be failing.
You CPU might be overheating due to a dirty or failing cooling fan.
Many other things. . .
This is an easier fix than you would think. This website (link) tells two easy things to do that speeds it up for me every time. This is the first thing I do to every computer I get when the owner says their computer is slow. Good luck!