Hi,
Its possible that there's a typo in entering the code...
Try it this way:-
Login to your Planet Auran account, find your serial number, highlight it, then copy to clipboard.
Create a small text file (Notepad is ideal), and paste the serial into the filr and save to desktop.
Install from DVD, but when asked for the serial number, copy and paste from the text file.
I've just reinstalled TS12 onto a new PC (old one died), and there is no problem with installing on more than one PC.
HTH,
Colin