Simple Log out windows authentication

 Hello ppl,
Today i come to you with a problem wich i cant seem to find a solution 2 although logically it should be simple.
I have a log out button, i just want it to log out from the ISA authentication when its clicked and return to the ISA logon page.
i tried :
Session.abandon();
Session.Clear();
Session.RemoveAll();
all that doesnt seem to do any good.

 Im using VS2005 and code behind asp.net c#.
Thanks in advanced.
 
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