Back to square one.
A fat white cross is the normal cursor for Excel.
Are you saying that if you select any cell then attempt to select a different
cell, the area between gets highlighted?
Are you holding the left button down and dragging the cursor?
Is Excel acting like the button is being help down?
What happens if you use the arrow keys to navigate?
I would try banging a little harder on the wheel a few times.
Gord