IIS 6 - IIS 7 Issue

I created a website using VS 2010 on an xp box with IIS 6 and am using windows authentication, but I have code that looks at Request.Servervariables["LOGON_USER"] and passes them on.
 
Problem starts to occur when I copied my site to my windows 7 box with VS 2010 and IIS 7.  Now it constantly prompts me to enter windows authentication.
 
Any help appreciated!
 
Thanks
 
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