Check the status of the program each time it runs and it should behave based on what it finds in the registry.
Bulletproof? No, but for this system to fail does take an experienced cracker.
I beg to differ. Such protection systems are unbelievably easy to remove by anyone with the slightest idea what they're doing. I've removed these sort of protection systems from games many times ( for personal entertainment, I hasten to add ) and I have very little idea what I'm doing.
If you attract the attention of such a person, there is no protection you could apply that would stop him.
Software Passport's highest level of protection has never been cracked.