My client connect to Access point and ethernet hub via first network card. I will manage this client and redirect to second network card (internet connection). İf username and password is correct.
There are several WiFi hotspot solutions based on WinpkFilter are available on the market (TrueCafe, Antamedia and etc…). In general you have to redirect user to authentication page on his first attempt to use Internet browser, then if user is allowed to use Internet you have to save user credentials (MAC, IP addresses) and route user packets to external network. The detailed description of this would take a large amount of time, but if you have some more exact questions I will try to help.