Unless you purchase a third party component; there is no built-in role management support in .net 1.x.
You'll have to implement the whole thing by yourself (which is really not that bad). There are many ways to handle it, but off the top of my head, this is what I would do:
1) make 2 menus using User Control ascx, 1 with "sign in " link and the other one with "log out" link.
2) in your aspx page(s), put a placeholder control as where you think the menu should be located.
3) store a username/password somewhere in your sessions, and default name/password to something bogus.
4) I assume you use FormAuthentication to manage logged in user, so you do the following under your page_load method:
//grab username/password from sessions...
if (FormsAuthentication.Authenticate(UserName, UserPassword))
con = LoadControl ("login.ascx");
con = LoadControl ("logout.ascx");