"Steam has a bad habit of not letting you choose where to put your games once it's installed, so dropping the app on your system drive is going to require you free up plenty of room for any Steam-based games you buy."
You can move your steamapps folder off drive, and create a link (using mklink) in the original steamapps folder to the off drive location, now all your games are on a seperate drive.
first, close steam, and move the steamapps folder to your D:\SteamLink\ folder.
Open a command prompt as admin (hit start, type cmd, then ctrl+shift+enter), and use the command:
"mklink /d "C:\Programs\Steam\steamapps\" "D:\SteamLink\steamapps". (if you have windows xp, you need to install a microsoft module to enable this command, consul the internets for help)
Once the link is created, start steam, it will be none the wiser as to what you've done.
The same strategy can be used to move apps you accidently install to your C drive to another location, to move users folders (this can be tough), and to move select apps onto your SSD (like this weeks favorite game).
I advise against moving anything from the windows folder in this manner, in some cases it can be done, but I've had issues with system updates after doing so.