Check for valid user name and password on domain

[Isabel]

I need to check to see if a user name and password is valid on a
domain. I can't use LogonUser because the user don't have a local
account setup. All I want to do is call some function and pass it a
user name, password, and domain and have the function return true is
valid or false if not valid. Any help on how this is done is
appreciated.