I've had this happen to me before. Turns out my hard drive was going out and there was a bad sector on my boot file (the part that tells Windows to load).
If you can get it working, you may want to look into a new hard drive, as it will probably happen again. I know there are boot discs that you can use to try and re-install this part of the hard drive and allow the OS to boot, but I do not have much experience with this. If you get a replacement hard drive, you may be able to use the new one, and pull files off of the old one if they are both showing up on your computer afterwards.
