The following works if a computer has a network card from which you can read the SN. The easiest way I know is to generate a new GUID and take from it the network card SN.

Of course, this number is specific to the network card, so, if the user changes the network card, your software will not work anymore. But this problem doesn't have a solution. Whatever ID you would use, the user can always change that part (or even the whole computer), so, you have to deal with this situation anyway.

Vlad