[AccessD] Registration routine for a commercially-sold Access app?

I need to come up with a registration/licensing routine for an Access
app that a client of mine will be selling.  Basically, my client is
concerned about one customer distributing the application to others.

I want to roll my own algorithm, but then it occurred to me there
might be a better way.  Right now I'm experimenting with validating
the app against the hard drive serial number, but that's fraught with
shortcomings.  I'm also thinking about using an obscure registry key. 

Who's had to come up registration methods before, and how did you do
it?

-Christopher-