Just release a cut down version (delete a few layouts from it) as your trial version. No saving anything to the registry or modifying ini files and uncrackable on account of it not containing the rest of the files to begin with.
Then when they buy a full version off you, compile a customized copy of it just for them, along with their payment/address details displayed within the game intro and no anti-piracy protection (the pirates aren't going to crack something that doesn't need cracking.. no sport), and if Joe Bloggs tries passing around their paid for version to the rest of the public, all their private details are made public.
It's not perfect, but it's the least amount of messing around. Course the data protection act might have issues with you including their credit card details in it.. Just say you left a briefcase with their details on a train somewhere, always works for politicians.