Problem clicking in cells

I'm using Microsoft Office Excel 2003 on a Windows XP laptop. I've been able to successfully use it for many months now but am currently experiencing a problem.

Whenever I try to click (left click) on a cell, the program basically acts as if I'm clicking and holding so when I move my mouse around, it begins to select multiple cells. Even when I move away from the cells and to the top menu the icon stays as the plus sign that is typical when hovering over cells and I'm unable to access any of the menu items (such as File, Edit, etc.). Eventually to get out of it, I need to push control alt delete and force quit the program. Once I've clicked on a cell, there is "no return". It just keeps selecting multiple cells. I've tried double clicking, right clicking etc. to no avail.

Any ideas?

I'm using a laptop and I've tried using the mousepad AND an attached mouse.

Thanks in advance.