Keyboard stops working

A user is typing along (different programs) when he hears 2 or 3 beeps, a squeal, then the keyboard stops working. The mouse still works, and logging off then on fixes the keyboard. It only happens 1 or 2 times a week (or less), so I can’t try logging on as a different user or have him log on to a different computer to troubleshoot. We have switched keyboards and mice, no change. I am considering giving him a new, default, profile. It is a Dell Dimension XPS, PIII 750, NT 4.0, SP 6, 384 Megs RAM. Any suggestions?