Good question and one that deserves a better answer than this. It is possible to select items in any screen using just the tab, windows key, Alt key and arrow keys. It takes some learning and the best option is to get the mouse working by doing what your friend did - reboot!
To help you do this remember Alt+F4 key shuts the active window. Use the tab key and enter key when prompted to save work to select Yes/No/Cancel. Use the Windows key to select the start button and then arrow keys to navigate to shut down. (In XP Windows Key + U + U does it)