Using authentication for certain sections of a page

Hi, I currently have a website which I want everyone to be able to view, authenticated or not. But if someone does sign in, I want different controls and different C# code to be run on the page.
At the moment I have not put any asp security in. I just have two text boxes that check against the database then keep the user logged in using a session variable. Each page then checks to see if the logged in session variable has a user in it and runs the code. 
I'm assuming this is not the most secure way, is there a way to do this correctly?
 
Thanks in advance. 
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