Just a GUESS... and I may be wrong, but try uninstalling the demo version & installing the regular full version, then try registering it.
It's how I did it when I first got it, and everytime I've reformatted my computer since, I've used a copy I made of it to put back onto my computer & just put my original registration information when it prompts without any issues.
www.youtube.com/evilmonkeylord
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