Removing program licence

Someone has previously installed and licenced a program on my computer (Windows XP) and then removed it. Now I have bought a licence and downloaded this program, but even after deleting every evidence of the program, it still uses the other person's licence and won't let me put mine in. Help! (program is Photomatix Pro)

What is the program?

The key is most likely encrypted in the registry.

The program manufacturer may have specific details for removal.