You can't encrypt an executable - it has to be unencrypted in order for the system to execute it, and any in-process code encryption would be pretty easy to break as it would have to include the decryption key in order to be any use.
Have a look at Obfuscation instead: it is about the best protection you can practically give - but it varies from company to company, and price range to price range. Google can help you:
Obfuscation software[
^]