Frozen Cursor

My desktop computer has been having a frozen cursor incident about once a day (it can happen at any time and has happend during scans (McAfee, defragging, etc). Ctrl-alt-del
doesn’t work so I have to do a restart on the tower. Are there diagnositic tools available to find what is causing this. I have XP Pro
sp3 with all the updates. I have run several virus scans (including MS, Symantec, TrendMicro) and have found nothing suspicious.
Thank you in advance, Bob K.