Copy protecton

[Sadhunathan Nadesan]

Aloha,

Can anyone help me with some ideas for copy protection?

I'm developing a small Windows application that I want to give
for free to existing customers, but I don't want them able
to use it after they are no longer customers, or, making
copies for friends.  I'd prefer something more robust than
a license key or password because that can be shared too.

One idea I have is make it web enabled ..  it has to check
back with our server and verify a valid customer record 
exists.

Another idea - put a time bomb in it.  When a certain date
is reached, it expires.

But if there is a simpler way, would love to hear it.

Thanks all,
Sadhu