A neighbour of mine had a problem with her mouse just randomly failing to work. We re-installed the software for the mouse and the O/S with no improvement and were starting to looking at the options of a new mouse or a new motherboard - it was a newish computer.
Anyways, we swapped PS/2 mice. Both continued to work without any problems. We switched them back a few weeks ago and the old problem returned!
Try another mouse.
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It's not easy having fun; even smiling makes my face hurt.