Hi,

For reasonably secure way to encrypt strings as a basic registration mechanism for a reasonably cheap software you may check this download section and look for encrypt.zip by Rohullah Habibi.
I dont know if it has a specific construction for calculating a message authentication code involving a cryptographic hash function in combination with a secret key but it looks like the alorgithm serves just your requirements: cheap and easy to implent!

Regards,

Koen