Personally, I would say you might want to run Command Prompt and run a chkdsk /f. That should fix any errors on your next boot.
If it is more than that, you might have a corrupt hard drive or, as Simon said, there could be a hidden partition. Just go to Control Panel, Administrative Tools, open up Computer Management and click on Disk Management. That will give you a list of all of your partitions plus allow the ability to tweak with them.