AD Session ID and authetication?

Hi!
I am working on an intranet where I still need to authenticate users wanting to access reports through a web interface.
I do not want that users should enter user ID and pwd again just to enter into the tool, as they have already authenticated when they logged on to the intranet. Somehow my web interface -client side -should be able to pick up the session ID and user ID that the user is accessing from (through an AD call?) and thereby my web interface can pass on these credentials to the server side that can then validate them again before returning any reports to the client. But how do I do this?
I know that AD and LDAP contains a lot on this, but I just do not have any experience with these Windows aspects. I am an experienced C/C++ programmer - if someone can give help, hints or sample code it would be just super!
Best regards
Gustav