Can you register the activex control manually? The only thing other than that I can think of is possibly a permisions problem on the one machine.

I have once seen a similar problem where we could not get a control to register. In the end after speaking to the clients IT guy he told me that the specific machine had never really worked right and that they had to keep reinstalling loads of applications on it. In the end they rebuilt the machine, just a thought to see if the machine has any history.

These are pretty wild guesses and you have probably thought and tried this already but cant think of much else.

Jon