Mouse selecting multiple cells and won't let go

I'm working with Excel 2003. Recently when working in a spreadsheet I can't
select a single cell without multiple cells. When I move the cursor with the
mouse a range is selected and won't stop. When I try to exit it won't respond
to mouse or keyboard commands. When I go to task manager and end task I get a
pop-up that says "Can't quit Excel". I tried the F8 key, the scroll lock is
not on and escape key doesn't work either.

Any ideas what this is and how to fix it? Thank you. Hill0882