I use Steam, Valve's digital distribution software for PC games, to play several games. I wondered if it was possible to install games to different directories or drives but couldn't find any suggestions. Steam would only let me install games to a single commonapps folder. I had no luck searching through the forums or with Steam support.
I stumbled upon a Windows 7 (and Vista) feature called "symbolic links" which solves the problem. You can move the game folder to another location and leave behind a 'symbolic link' to the data.
Hermann Schinagl authored a great tool that makes creating symbolic links a cinch using Windows Explorer. You can find it here
How to use:
Step 1: Copy the contents of the folder c:\...\steamapps\common\plants vs zombies\ to a newly created folder on a separate drive d:\...\plants vs zombies\
Step 2 -- Select the newly created folder in a 2nd drive d:\...\plants vs zombies\ in Windows Explorer. Right click. Select "Pick Link Source".
Step 3 -- Navigate to your SteamApps-->common folder c:\...\steamapps\common\ Right click in empty space. Then select Drop As-->Symbolic Link.
Voila! Your Steamapps\common folder will have a new folder w/ a link icon called "plants vs zombies" which seems to contain all the necessary game files. Except the files are now actually stored on a separate drive/folder of your choosing!