|You cannot alter the C drive from within.|
To alter the partition structure of the disk you will need a partitioning tool such as Partition Magic run on a bootable CD.
I do not particularly recommend that tool over others that are available since I have no hands-on experience with any of them. They all however come with this warning.
Back up all your important files to an external location before you do anything, and test the backup. Then be prepared to re-install from scratch as altering your operating system partition carries a significant risk.
If it were me doing it I would wipe the drive completely and then create new partitions of whatever size required, then do a fresh clean install of the operating system.
Goin' Fishin' (Some day)