Left-mouse keeps quiting

I am supporting a remote user whose left-click capabilities keeps dying. If he hits CTRL+ALT+DEL, he gets the use of his left-click back (for a while). This is affecting both the touch pad on his IMB ThinkPad (running XP Pro) as well as the USB mouse attached to it.

I have uninstalled all mouse drivers and reinstalled the appropriate UltraNAV driver for his touchpad. I also ran CCleaner in hopes something in the registry was wigging.

Any help or suggestions greatly appreciated!