HTTP Error 401.2 - Unauthorized

I'm trying to implement Forms security. So, in IIS, at my website level, I have disabled anonymous authentication and only have Forms security enabled. As far as I understand, I shouldn't be able to hit any page with the exception of the login page. This is the config:
<authentication mode="Forms"> <forms name=".ASPXFORMSDEMO" loginUrl="/login.aspx" protection="All" path="/" timeout="30" /> </authentication> But, when I try to hit the login page, I get this:
HTTP Error 401.2 - Unauthorized You are not authorized to view this page due to invalid authentication headers. Why?
I've tried following this link, but as mentioned, I want to use Forms authentication, not Windows, not anonymous, and not anything else!
http://support.microsoft.com/kb/942043/< ...