how do I manage ascx files in an aspx page?

Hi

New to asp.net web applications so bear with me please....

I have a dafault asp.net page called default.aspx. It has a couple of ascx's in it. One for the top banner which I want on the site permantly and one containing the login name and password (this contains the vb file with the login code in it that instantiates my clsUser class and tries logging in to the database). On logging in successfully how do I display another ascx in the place of the login.ascx ?

Also, once logged in and having redirected the login.ascx to, say, main.ascx (but keeping the page displayed in the browser as default.aspx), how do I keep the instance of clsUser.vb that I instantiated on logging in in the first place so that I can refer to it later?

Cheers