Uh, spend the $5 and get a new mouse. It's either that or the PS2 port on the board is dead... which would be more trouble to fix than it's worth. If a new PS2 mouse doesn't work, just use a serial ...

I've got a USB mouse (MS) that came with a USB to PS/2 converter. It works fine hooked up to a PS/2 port. I run it through a KVM switch that is PS/2 and not USB so this is necessary. One of the boxen ...