Almost all has already been said, but I guess I can add just a little bit more advice.
Two years ago, I wrote an alternative to LogonStudio Vista to learn WPF and keep my C# skills up to date. It supports almost all the logon screen elements (buttons, overlays, start orb animation, sound, etc.), runs as standard user with only a small process running as elevated to actually do the logon changing work and even has support for logonstudio XP & Vista files format via plugins.
Without even having to try your program, this gives me a very good idea of why you do not support Vista at this time...
Now, even with my own program having more features than LogonStudio, I would never even try to call it better. Those things are hard, it involves doing stuff you're not supposed to do. And so, in some cases there is a good chance you could render people computers inoperable (not completely useless, but just restoring from the last known good configuration is already too much for most people). LogonStudio has one crucial feature: it has been tested and deployed on millions of computers. And that's something you just cannot replace.
You're only 13, and learning how to program and getting a head start is a very good thing. But you still need to learn something very important if you want to be a successful businessman and even more so a successful programmer: humility. A lot of people know a lot more than you do, probably a lot more than you can ever learn. So try to find those people, just sit and listen, and try to learn every bit of knowledge they'll throw at you. Believe me, this will make you a better person.
And maybe some day, people will come to you to ask questions