1. Try repairing it.
2. If that fails, I think the Windows CD will have a repair option. This should be a good bet to work.
3. If it fails, and if you have a backup of the OS drive then try restoring it. (This will lead to losing applications and data on drive since the time backup was done)
4. If that fails, take it to support and let then try to fix it.
5. If they fail, than you would probably have run out of options and have to resort to do a fresh install.