It looks like your drive got corrupted.
It could be only a single key file that is missing, or it could be that the drive itself have gone bad and need to be replaced.
Get a Windows install disk or a Linux live CD and use it to boot the computer.
Check if you can access the drive at all. If you can access the drive, copy all your personnal and collected data to an external drive.
Once your data are safe on another drive, try to do a repair of Windows. If the repair don't work, you'll need to reinstall Windows.
If you can't access the drive, get a new one and install it in place of the actual drive. You now need to install an operating system from scratch.
There still some chance that you may be able to recover your files, but you should not count on it. If there is something broken on the drive's controll board, the data are still there but the drive's motor may not even run or you may no longer be able to direct the drive heads to read it.
Do a search for some data recovery software. There are several, but most will cost you some.
BEFORE you pay anything, take your time to read any users revew, comments and feed back you can find.
Electro, May 2012
