Alternative: mounting a different partition as a folder on another. This is easier and slightly safer than rearranging your partitions, but the resulting configuration is not quite as versatile.
Create a new, normal partition in the unallocated space. On your C drive, choose or create an empty folder whose disk space will be provided by this new partition. (Your Documents folder might be good a candidate if you can temporarily move all the stuff out.) Right-click the new partition and choose Change drive letter and paths.
Click Add.
Click Browse, then navigate to the empty folder you just made or cleared out.
Click OK on all the dialogs. Changes take effect immediately.
The disadvantage of this solution is that only files in the directory you choose will be stored on the new partition; other locations on the C drive will still be subject to the size restriction.
diskmgmt.msc
to get there directly.unallocated