First try physically swapping the HD's to see if either will startup on the other machine. I've had good experience moving Windows 7 HD's to other machines. It will swap out all the drivers
, request several restarts, then work fine if it starts in the first place.
Several have reported that they didn't even need to change activation, however I have always immediately changed out the key at Computer>Properties link to refresh it on the new hardware. If you needed to do more, you can run a Repair Install
which resets the activation and fully acclimates to the new hardware.
Before trying this, I would create a system image of each HD saved externally, so if necessary you can reimage it to the other machine's HD. You can even save the image in a primary partition on the same or secondary HD. The booted DVD or Repair CD will detect it there to reimage via network to other HD.