How do I check whether a user is in an Active Directory group when given its username and password

[C# Programming]

I need to check whether a username and password are valid, and if so, if that user is in a special group on Active Directory.

I'm developing an intranet only system that (hopefully) will allow a user to login at program startup using their Active Directory credentials.

If anyone knows any C# code that does this it would be greatly appreciated. I've done a thorough Google search, but haven't found anything that returns a helpful response.

Thanks