You want to "reallocate" space from your game partition to your OS partition? I don't think it can be that easily done. How big is your C: partition? Maybe just some cleanup using CCleaner, moving some of your "Document/Download/Music" folders and moving some software would be a simpler idea.
A utility like Partition Magic can do things like that. The first thing you should do before trying though is back up the drive, as a power failure or other glitch during the process can mean lost partitions.