C# - Replace Defaut Authentification Module?

Suppose I've written my own authentication module implementing IHttpModule. It is not really done but there are good resources so I can figure it out.

There are also tutorials on how to add custom created module to Web.config. But how can I replace some default module ? That is how do I remove default authentication module from execution path ?

Can I just remove following lines from Web.config?

<authentication mode="Forms">
<forms loginUrl="~/Account/LogOn" timeout="2880"/>
</authentication>

If I remove them will the default authentication module still work ? Of cause there is no real need to remove that module even if I don't use it... But it'll save some processor time and I'll feel better.

That was easy, not the hard part: can I replace HttpContext implementation? I have my own session class called Session and I don't want that HttpSessionState to be in HttpContext. I'd also like to replace User property of HttpContext. So I guess I need to create my own implementation of HttpContext and force my aplication to use it somehow. How do I do it ?