|if you can download this and create a bootable cd:|
boot from the cd and run it for a few hours. If the mouse problem goes away, reinstall xp. Do you have the xp cd that came with the computer? A repair install could be an option and if you have the cd, it won't cost you a dime. you never answered the question, did you try another mouse? Is the mouse 7 years old? are kids using the computer?