It sounds like this is a case of the hard freeze (aka: Num Lock won't even turn on or off on the keyboard, aka: the computer wants to Blue Screen, but it can't even do that).
Start checking for hardware issues:
1) RAM.
Download Memtest 86+ from
http://www.memtest.org. Burn .iso to a CD. Boot your computer from the CD and let 'er rip. If there's ANY red on the screen after a pass or so, then you've got bad RAM.
2) Hard drive.
Run CHKDSK against the drive. Since Windows 7 doesn't have a recovery console, you can do one of the following to get to a command prompt and run CHKDSK:
- Run CHKDSK from the command prompt while in Windows. You'll be asked to have it run upon next reboot. The problem with this is that you won't be able to keep the results on the screen if you're not watching it at the very end.
- Run CHKDSK from a recovery disc. To create one, go to Start -> All Programs -> Maintenance -> Create a System Repair Disc. After burning this, boot from it, go to the command prompt, and type in the following, pressing Enter afterward:
chkdsk c: /rAfter it's done, check the number of bad sectors. If that number is NOT zero (0), then you've got a dying hard drive. Back up your data and get a new drive before it's too late!
3) Expansion cards:
Start removing them from your PC, one at a time, to see which one is the culprit.