Drifting Mouse Cursor

This just started. I’m running a windows XP system with 320 megs RAM, 450mhz processor and 60gig HDD. My cursor without warning just started drifting all over my page. I’ve done a virus check in safe mode found nothing. Ran Ad-Aware, HiJack this and CWShredder. Found nothing. Any suggestions as to how to fix this problem. Rather annoying when getting ready to click it drifts off intended click and runs all over screen. Thanks