Mouse cursor has started sliding down off the screen continuously

I have a Dell XPS 8300 (12 months old) with a Dell wired mouse that I was using last night.   I did not visit any unusual websites, nor open strange emails, nor does McAfee have any threats showing.  The mouse was functioning normally during several hours of gameplay.

At about 11.00pm, I closed several applications (Rise Of Flight, TrackIR5 and Teamspeak3) that I have run together for at least 6 months without problems. When the Windows7/64 desktop came back in view, the mouse cursor started sliding downwards and disappeared off the bottom of the screen.  I could drag it back up, but it was continuously sliding down. 

I am typing this at work, not on the home desktop, because the home PC is dysfunctional without a working mouse.  The efforrt to match the unwanted slide downward with a physical mouse slide upward means that the left-clicks miss or have too much movement for them to work.

I unplugged the mouse, and the PC reloaded the specific mouse drivers = no change;

I tried the mouse in a different USB slot, and the PC reloaded the drivers = no change;

I replaced the mouse with the one from my previous Dell, and the PC loaded plug and play drivers = mouse functioned but cursor slide continued.