On the server, which HTTP daemon software and OS are you running? On the client PCs, which OS are they running?
What you're talking about is commonly referred to as a Single Sign-On or SSO type of login. For Windows clients, you'd want to look into using NTLM authentication.
EDIT: You really need to elaborate on what you're trying to do... are you asking how to take a given username and password and verify it against... something? If yes, what is that "something" ? If no, then what are you trying to do?