Your keyboard has probably become defective. Check with some other keyboard whether the problem persists or not.
I recently reformatted my master hard drive and re-installed Windows XP Pro. I also installed Linux (Ubuntu Feisty Fawn GUI interface) on a slave drive. I can boot to either the Windows or Linux OS but now whenever I press the up arrow on either OS the computer shuts down and I need to restart it. Any idea on what I need to do to re-program the up arrow key to function normally without shutting down the computer?