I came across this problem two days ago. I started up my computer (Windows 7 Professional SP1, 64 bits / 4 GB, Intel Core i5 650 3,2 GHz) and it automatically signed in to one of the users. The shared computer got 5 accounts (three of them got a password) and it automatically signed in to one of the accounts without a password. Then it just stays there, it's not frozen but the computer doesn't respond to my keyboard and mouse (and yes they are plugged in). I tried using Ctrl+alt+del without success. I can only turn off the computer by holding the power button of the computer itself.
Any help would be appreciated and feel free to ask me any questions if things are unclear.
untick the above answer until your problem is fixed
Tap F8 as you first power on and boot into safe mode with networking se if the keyboard and mouse are working then. If the keyboard is not working to accept the F8 key then boot to BIOS and enable USB legacy for DOS - save and exit And retry.