By convention, the C drive contains the currently running Windows OS. As such, it is not possible to format or delete this drive/partition from within Windows itself. It is typically formatted during the OS installation.
If you have managed to install the OS on another drive/partition (which you say you have), you should be able to delete/format this partition. Just check which letter has been assigned to it (which won't be C, as this is reserved for the currently running OS).