I finally determined that the problem was with the BIOS. (It had nothing to do with the particular USB port the mouse was connected to). Up[censored] the BIOS turned out to be more complicated than you would think. Normally you would create an MS-DOS bootdisk, load the BIOS onto it, reboot and proceed from there. However, creating a boot disk for XP for this sort of task is quite troublesome. I never did find a satisfactory bootdisk although I had a lot advice from knowledgable people. Fortunately, the CD-ROM that came with the motherboard (GigaByte) had a utility for upgrading BIOS that operated in Windows. It automatically detected the type of motherboard, downloaded the correct BIOS update through the internet, and installed it. Sweet.