Resolve username after login (DOMAIN\NAME)

HI,

I made a secure part in my site using windows authorisation.
Through the IPrinsipal i can read the name of the user.

I wan't to connect this to the active directory (.NET 2003 Server)
Also for new registrations i want to be able to add accounts to the active directory.

Any idees?
I can't find out how the DirectoryService namespace realy works :(