I suggest you don't even attempt to add copy protection. You will just annoy people. You are much more likely to get sales if people like your product. Being petty and treating customers like thieves is not going to help. (Unless the product really sucks, and you have to prevent people from being able to try it before they buy.)
You might want to send a license key the sole purpose of which is to satisfy users that want to receive something for their money. Just add a status somewhere to show whether it has been paid for. The status needs to be reasonably obvious so that it is clear that something has happened (a product I worked on along time ago came on floppy two discs - half of the support calls were because the installer didn't ask for the second [win32s] disc).
