You cannot format the partition which is currently running Windows 7. How would it run to do this?
Are you planning to reinstall the OS? If so boot the Win7 installer, choose Custom Install, then use the Drive Options to format C before choosing it to reinstall. You can also delete any other partitions you don't want, even C, to repartition as desired.
If you'd post back a screenshot of your maximized Disk Mgmt drive map and listings, using the Snipping Tool in Start Menu, we can advise you how to do most anything you want to do with your partitions.