Mouse stops working in Bootcamp (XP)
Have you tried using other mice, either the same make and model or another brand altogether, and do they exhibit the same problem? If not, then you probably have a hardware failure with your mouse.

Beyond that, what have you changed or upgraded or installed on the Mac side just before this problem appeared? What have you changed or upgraded or installed on the Windows XP side just before the problem appeared? There must be something. This is the key to troubleshooting.

Reset the PRAM.

If that does not help, report back.