I found this in a few different places around the 'net - hopefully it's helpful:

Use Your Keyboard if Your Mouse Goes Dead

If your mouse is not functioning, don't panic. You can use your keyboard to move around the Windows XP desktop. The keys to get used to are the Windows key, the arrow keys, and the Enter key.

• Press the Windows key.
• Press the up arrow key one time to highlight Shut Down, and then press Enter.
• Press the up and down arrow key to select Shut Down from the menu, and then press Enter.
• Use your Esc key to cancel.