Use some HD/partition manager in Windows, like Easeus
and format that free partition with NTFS file system. Then add it as a folder within C drive. Name new folder something like "Program Files 2".
Or you can extend partition, but then to increase partition size that space has to be available right after it because the shrink function operates at the end of the partition. In this case if it's not a problem for you, install Ubuntu on that free space ( make a logical partition ) and format current Ubuntu partition with NTFS ( also make it logical ). Then extend C. If you choose this option, first install Ubuntu on the free partition, then you can save/copy whatever you want from "old Ubuntu" to newly created partition.
AFAIK and I do now something about these things, those are the two best things that you can do.