How to protect my application so only the person that has the key or password uses it

Depending on the uses of your program, internet registration may be the best way, however if you program does not require an internet connection to work then you should avoid this.

I would say that the MAC would not be a good idea for a conventional household application as network adapters change from time to time and the active network adapter changes frequently on laptops. This only matters if it is checked every time the program is run, this would imply that the user is connected to the internet on the same interface every time. (this means that they can't change between wired and wireless on their laptop)

Unless you think your program will be widely used, a conventional serial number based off of something in the users computer (this could be the MAC (good for businesses), CPUID, or Windows product key (good for a household programs)) that is checked once upon installation, and not verified when running (or perhaps verified on the first run only) should be enough as it wont likely be reverse engineered or cracked.

Also, if the program is intended to be used by other businesses, then you should not need any copy protection. Businesses will not risk getting caught on software licensing violations because of the harsh penalties (might vary in other countries).

Just about any protection scheme has been broken - maybe you should consider providing your software as web based application. This way you will keep the binaries out the hands of enterprising minds, and possibly build a better business case. If you don't like HTML, MVC, asp.net - you should take a look at SilverLight.

Regards
Espen Harlinn

Another possibility (not saying you should go for it, but just consider it) is using a hardware dongle that your customers would plug in their USB port. Your application works only if the dongle is present. Such dongles are nor easy to counterfeit, but then you can get into issues with the logistics of sending out the dongles.