"Licensing" utility for protecting my Excel code
 

I would like to make some worksheets with VBA and cell
formulas available to peers in my profession, for a
nominal fee.

"unlicensed" users would be able to view or print the
workbook, but not change it. "licensed" users would have
full use of it.

I don't want to use a password, because passwords can be
shared. I want to "license the workstation".

I have AutoCAD tools that look for a "license" file before
they're executed. That file stores the workstation serial
number and an authorization code. If the information
matches, it allows the tool to execute. It's very simple
(and, probably, easy to break), but it's effective.

I'd like to do something similar with my Excel tools.
Does anyone have any suggestions to point me in the right
direction??