If you want to use a P/S2 mouse (small circular plug) you can enable the P/S2 port in your CMOS. It should be detected by Windows, and then you will be good to go.
Normally, it uses IRQ 12, and will free-up your comm ports, as the hardware for P/S2 is built into the chipset on your motherboard. Be aware you may endup with conflicts if something else is using IRQ 12 now.
Ray