There are many threads here already on this subject.
Search for the term 'Mac address' you will get over 122 results most of them on the subject of security.
and try the following specific post
http://forums.ni.com/ni/board/message?board.id=170&message.id=102353&query.id=23683#M102353
look for the notes by Enrique
You could of course buy a hardware key as well....
