A serial mouse, sounds like you machine pre-dates Christmas ! :?
Since I don't know your OS, or system, here's an old article that might help :
http://www.pcguide.com/opt/opt/resCOM-c.html
It basically talks about IRQ assignments for various ports, which may conflict if not set up correctly. This may get your PS/2 mouse a life. It's a long shot, and for the life of me, I can barely remember using a serial mouse, even a PS/2 is starting to stretch it a bit.