Symptoms:
Clicking any cell in Excel highlights everything from that cell back to
A1xA1. Moving your mouse behaves like the left mouse button is being
held down (or stuck). You need to Ctrl+Alt+Del (CAD) to get out of the
document and close Excel altogether.

My Solution that worked:
Ensure mouse is operating fine, not dirty, or sticky keys.
Ensure EXT mode is turned off (F8 in Excel)
Ensure Scroll lock mode is off in ControlPanel Mouse
Installed the latest IntelliPoint Drivers (5.2 at time of writing)

And Voila! That should fix it.

Joel