Excel selects multiple cells when I only click once (and then I can't do anything)

In Excel 2002, when I click on a cell it acts as though I am holding down the mouse button and continues to select cells. Then I can't get the program to do anything at all... I can't even select anything from the menus. Pressing Escape doesn't do anything and the program is totally useless once this happens.

 

It used to be only an occasional problem which would just happen apparently at random, but now it happens almost every time I open the program (even on a new blank spreadsheet). So at this point, Excel is virtually useless. The only way I can do anything is if I completely avoid using the mouse/touchpad and do everything using the keyboard.

 

Anybody know what causes this and how I can fix it? Any advice would be greatly appreciated!

 

Thanks.