hmm....which kind are you using, ball mouse, optical mouse, or laser mouse?
if it's wireless, check for interference from a radio or something, I've heard (rarely) that that can interfere with it. Also, try checking the batteries, cleaning it, things like that. Hope that helps!
the old system...
EVGA GeForce 6600GT
Enermax Whisper II 535W PSU
Chaintech VNF4 Ultra