Identifying a certain Windows machine

[Joe F.]

Does anyone know of a way to identify the Windows machine of a certain  
user in a reliable way.

I want my app to download a stack file which will only run on one  
user's machine.
Is there a way to access something like a MAC number or some unique  
identifier on Windows machines.

Bonus question:
Is there a good way to make a stack self-destruct?

Cheers,
Joe F.