Hi there! Thanks for your response.
I just learned that when the mouse is locked up, the Windows key doesn't function, either. The rest of the keyboard does.
So, I checked the Event Viewer after I locked the mouse up again, and switched users to unlock it.
The only error listed was this one (from two hours ago):
"The device, \Device\Ide\IdePort0, did not respond within the timeout period.
For more information, see Help and Support Center at [url="http://go.microsoft.com/fwlink/events.asp.""]http://go.microsoft.com/fwlink/events.asp."
That doesn't seem to be related...?