In my experience, rarely does anything on a puter go 'wrong' without a little human intervention. Can you think of anything you have changed, downloaded or installed inbetween your mouse functions working correctly and incorrectly.
If so try uninstalling them. Failing that if you are using WinMe or later use the system restore funtion (found from the start menu) and restore your system back to a time when you know all was well.(remembering that you will technicaly be losing anything you have installed after the point you restore to).