Activation Keys

I'm wanting to have my program have a Activation Key, like a HashCode, tied into a couple of settings.

For example I had the thought of taking the two Settings and making a string from then using md5() on that and that would be the Activation Key. The problem is that I'd have to have in the program the code to actually generate the whole Key anyway in order to compare the one they give with it. And then someone could extract this given time and effort.
I'll go with that if I can't find a better way but I was hoping to be able to find a way in which my algorythum used to verify could do so without creating a new key.

Use some hardware, and run an encryption on it. Store the value in the registry and each time you run the program, read the registry and compare the encrpted registry key with the encrypted hardware key. martinliss has an example using the hard disk serial number, but there are other methods that can be used.
i use this method to also generate a back-door password in case they forget a manager's password that they choose.

the pc numbers are emailed to me before they can run the app, but you could probably have some trial period, but i didn't want to, as it could be defeated.

I've found this in the code bank https://www.vbforums.com/showthread.php?t=231798 and mixed it up with md5 and it seems to be ok, still generates the key in order to compare but i guess it'll have to do.

I've tied it into some Key Settings on the system, if they change then the likly hood is its being run by someone else. So they'd need a new code, which if i believe its still for them i'd be able to issue the new key.

Well, I changed my equation, so that only I know who it is. The odds of anyone else wanting a code is pretty low, so I'm not too worried about it.