I'd probably go with:
C: System (Windows and programs)
This keeps the bulk of disk activity separated so you're rarely reading and writing to the same drive at the same time.
What Jim said... and what I do... (If I had 4, I would do as Jim says)
My 3 hard drives for video editing are configured as...
1 - 320Gig Boot for Win7 64bit Pro and ALL program installs (2)
2 - 320Gig data for Win7 paging swap file and video project files
When I create a project on #2 drive, the various work files follow,
so my boot drive is not used for the media cache folders and files
3 - 1Terabyte data for all video files... input & output files (1)
(1) for faster input/output with 4 drives
- use drive 3 for all source files
- use drive 4 for all output files
(2) only 60Gig used, for Win7 & CS5 MC & MS Office & other smaller programs
Search Microsoft to find out how to redirect your Windows paging swap file
Drive C space http://forums.adobe.com/thread/1007934?tstart=0