You may have corrupted files or a bad hard-drive, or possibly another hardware issue.
See if the BIOS has a disk check utility to run. If not, download and burn a rescue disk like Ultimate Boot CD or Hirens and run the drive scan utility from there for your hard drive model.
If that comes back clean, you may need to run the restore to get Windows working. That will wipe the disk and set it to factory defaults so you'll need to restore your files from a backup. If you don't have backups, remove the disk and copy your files into another computer, then run the restore.