That is easily possible with
http://www.rekenwonder.com/linkmagic.htm .
Download and install the program. When it asks you if you want to scan you can safely say no.
Now you need to do some preparation:
Create an empty folder at D where you want your games to land.
Close steam and navigate into it's folder. Either delete or rename (recommanded) the steamapps folder and create a new, empty one.
Back in Junction Link Magic click the create button. In the left window choose the empty steamapps folder, in the right window the new one on C.
Move the content from the renamed steamapps folder into the new one and start steam again. (This will take more time than usual moving, since you really move it to the other partition)
Steam shouldn't notice anything from this, for it it still looks as if the steamapps folder is a subfolder and not a symbolic link to a entirely different partition.

Alternatively if you use Vista or higher you can use the command line tool mklink, but that's a bit more complicated.
(Without garanty or responsibilty of course

)
Edit: Nice side effect: If you ever want to reinstall steam, remove the junction link BEFORE you uninstall and and recreate it after installing again and the games are
still alive!