No, it's FAR from simple.
If you want to do this on Windows XP, you'll have to replace the GINA with your own. You're NOT going to write a new one in C#.
On Windows Vista and above, the security model has been completely overhauled, so the same code won't work on Vista and above.
Seeing as Windows XP support is competely dead as of April 2014, I wouldn't waste your time with it.
There is no way to get your existing code to do what you want. You better read
] to see what you're in for.