I would vehemently disagree with jmiller47. 12 GB is fine, if the server is managed appropriately. The only things that belong on the C: drive is the OS itself and the program file binaries - data, software for download/installation, etc does NOT belong on the C: drive.
For more information on how to clean up space and what you can move/remove, please see:
http://www.lwcomputing.com/tips/static/bootdrivesize.asp