or resetting your wireless mouse. (I have a small wireless Logitech mouse at work and it has a reset button on the bottom).

Try those two and see if your problem still exists.
If it still happens on a wired mouse - you could try changing the way users log on (to the option of them typing their user names)

If it doesn't happen on the wired mouse but still on the wireless after you reseted it, try changing the battery. If no luck still then maybe it is time for a new mouse??

Good Luck.