|My only guess would be two things.|
One is that somehow in bios a legacy setting for mice might have changed. (dunno how it could really)
Second is driver issue with old mouse.
See event viewer first. See system hardware and expand hidden.
Might try a system restore to time when it did work.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10