Unlike in Windows XP, you cannot move the whole user directory to another partition, for example D:. Windows 7 makes it more complicated, but not impossible, to do this:

1. Boot your Windows 7 installation disk into recovery mode, then start a Command Prompt.
2. Logoff from your user account and login as local admin
3. Move the users directory to the data partition with robocopy. Make sure the Users directory does not exist on the destination drive before the copy.
   robocopy /e /xj /copyall C:\Users D:\Users
4. mklink /j C:\Users\username D:\Users\username (choose any path instead of D:\Users\username)
5. Enter dir to verify the directory junction has been established, exit the CMD prompt, reboot the machine
6. If you add files to the profile which where not created under the original user, you need to set ownership of the folder and it's contents to yourself (right-click the folder and click properties, then advanced)