It seems that MS disabled that key in Windows 7, they don't like when people tinker with some parts of the registry (the ones that you are not supposed to mess with it):
The long and sad story of the Shell Folders key
Usually you shouldn't change the location of any system folder just changing a registry key, sometimes it just don't work and other you could break the system, see:
Microsoft does not support changing the location of the Program Files folder by modifying the ProgramFilesDir registry value
Moral of the story: if Windows doesn't offer a method for changing the location of a special folder (like you can do for example with Documents or Images), you shouldn't touch it.
For your case I would try the following:
- Move all your data (documents, videos, music...) to a external hard drive (I suppose that Windows and installed programs will be far below than 80 GB).
- Make an image of your primary disk (with Symantec Ghost or any similar program) and restore it in your SSD disk. Note that you can restore an image in a smaller disk as long as the size of the data is less than the size of the new disk (in Ghost you have to use Backup partition instead of Backup disk).
- Replace the primary disk with the SSD disk and see if Windows start correctly from it.
- If all works fine (it should, swapping disks is very common in IT), install the 500GB as your secondary disk.
- If not, just put your old disk as your primary disk again, and then complain about MS forcing you to reinstall Windows in order to take advantage of your new disk.
I this way you would have also Windows in your SSD disk, which is better than having only Program Files.