Handling AJAX Login using C#.Net

Question

Hello, I'm having a problem figuring out how I am going to do this. From my C# windows application I need to retrieve a web page which contains a login form. I then need to proceed with logging in but this isn't a regular login form. This page uses AJAX to do the authentication and if it fails, uses GetElementByID to input data to the form (Ex: "Invalid User/Pass") in which I need the program to recognize. What is the best approach to achieving this?

Thanks in advance.