I tried to play Subject 66 a few days ago and that writes a file to the hard drive and windows said no way. I had to run in administrator mode to get it to play I'm sure it would do the same thing for registry keys.
Vista and 7 allow the application access to certain parts of the registry and disk. For example, an application can write to temporary files without security privileges, but not your Windows directory.
If the application steps outside this boundary, Windows hits the user with a UAC prompt. You just have to know what you're allowed to do, and you can avoid it. It's fully documented on MSDN, Microsoft want everyone to know what these limitations are so applications are more secure.
Then when they buy a full version off you, compile a customized copy of it just for them
Try that when you're selling 10,000 copies.