Hey, i have the same problem with my computer running win 7 ultimate x64. it was just happening, and a few things i found out. If you type in your password it lets you log in, but the desktop is not normal, but you can still access the ctrl alt delete, pres down 6 and right 1, which shuts down the computer without problems. and when i turned it back on, it worked! By the way, sometimes it works, like one of every 5 times restarting by holding down the button, so it could have been that random one, but it might be worth a try, for a temporary fix.