Well Clive, I can't say I've ever had the pleasure of that problem with my optical Intellimouse Explorer, but you could try searching for a driver update. Also, if it's a mouse that you had before you upgraded, it might be worth checking for a more XP-compatible update to the mouse software, too.
Try it on another machine and see if you can break that one.
If none of the above works, I'm out of ideas. Good luck!