Can I install windows on one HDD, but have users on a different one?

This is actually quite simple, using symlinks:

1. Install windows as you would do normally.
2. Move the Users folder to the desired destination.
3. Make a symlink to the Users folder. Run mklink /D "C:\Users" "G:\Users" in the commandline.

Repeat step 2 and 3 with the Program Files folder.