On every page (sitting in the master) I have the login fields.
In the other pages I have different controls, e.g. a contact formular.
How can I set more than
ASP.NET Web Forms are built around the concept of 1-form only. To get around this you'd need to either use a different flavor (a la MVC) or take a look at this article for how to get around it in Web Forms:
http://www.codeproject.com/KB/aspnet/CHtmlForm.aspx