It is not happening because it was a trial. We want people to trial first before buying, this ensures the software is what they want before buying.
Your computer is probably a little messed up if other software does the same thing.
When you enter a registration code into a software product, it usually gets written to the Windopws registry somewhere, so it seems that you OS may be having trouble writing to the registry.
Are you logged in with an account that has administrative privelages?
If not, install from one that does, or make yours an administrator.
You may also want to use some tools like www.spywareterminator.com
(free home version) to clean the pc up. Then run a registry cleaner like RegSupreme Pro at www.macecraft.com
Last resort-----save your documents and music and stuff to cd or a network drive and format that beast
If you do not know how, consult a professional who can help you out in reinstalling windows.