Is it a Hewlett Packard?
If so, chances are that the drive will be split into two partitions with a "mirror" of the factory installed OS on there in case a recovery is needed.
I think you press F2 or something at the setup "splash" screen, it gives you the option to perform a system restore. It wipes and reformats the original C-Drive with the information on the recovery partition.
All HPs are now supplied like this instead of supplying recovery media.
It saves the manufacturer money.
Don't forget if you do this you end up losing all your data on C so make sure that any important info is copied to an external drive or similar so it never gets lost.
Hope that helps mate.
