If ever your mouse stops working midway while working on your PC, the Mouse Keys feature can be a savior.
In Windows Vista, go to Control Panel, Ease of Access Center and in the second panel of the dialog box that opens up, you will find a section "Control the mouse with the keyboard". Select the check box that says "Turn on Mouse Keys" and then hit the Apply button at the bottom.
Now you can use the arrow keys in the numeric keypad of your keyboard to move the mouse around the screen.