You can change the location of your program files directories in the registry in the following keys.
Then reboot your system.
However being a software tester as a profession I would recommend DON'T DO IT.
Changing the location should work if you can assume that no software has any hard coded paths in the application.
Many, heck most applications use hard coded paths, so this will lead to a very, very unstable system.
I also find that partitioning a disk simply isn't worth the hassle. You'll typically run out of space on your C drive, or have chunks of free space spread across the drives you can't use, and if the disk fails you lose it all anyway.
Here's a Link to a Microsoft KB article on the subject http://support.microsoft.com/kb/933700