Validating Local Users, Passwords, and Groups

[krisk]

I have a question or two regarding local users and groups.

I have an ASP.NET 2.0 (C#) application (intranet-based, not on the
Internet), and, on one of the pages, I have two textboxes for end-users to
enter a user name and password. The user name needs to be an actual user
that is on the local computer, and the password (obviously) needs to be that
user's password. Sounds easy enough so far.

Now, the PHB wants validation of the entered user name and password (i.e., a
change in the requirements of the page).

Thus, when a end-user types in a user name and submits the page back to the
server, I need my C# code to take that entered value and validate that the
user they entered is truly a valid user on the local computer (the web
server).

So, my first question is, how can I take the value entered in the user name
textbox and make sure that it is a valid user on the local computer?

Also, once the user name is validated, the password then needs to be
validated to make sure that the password entered is the correct password for
the user.

So, my second question is, how can I take a valid user and an entered
password and validate that the password entered is the correct password for
the user?
Thanks!