More information please.
What kind of mouse? Mfg. Name, Model Name, Model Number, interface (USB, usb/wireless, bluetooth, PS2, etc.,), current driver if any installed?
What kind of system? Desktop, laptop?
Have you tried other mice?
How long? Has it ever worked? Just recently? Does it work on another system? Have you tried other ports? Is it connected directly to the PC or through a Hub?