Integrated Windows authentication for CMS website

Hi again,

I am trying to enable Integrated Windows authentication for my CMS website

so that users of the intranet do not require to log in on the website to
edit 
content...  Problem is that when I use a DNS name i.e 
www.mysite.richardsite.org, the authentication fails (I keep getting a box

asking for my username and password).  

Does anyone have any suggestion as to why this is happening ? Thanks