Windows Authentication working in Debug but not when published

Hello

I have a web application written in VB (VS 2008). I have the following code in global.aspx:
If txtReader.GetElementsByTagName("Security").Item(0).InnerText = "Windows" And User.Identity.IsAuthenticated Then
    authUserName = User.Identity.Name
    x = InStr(authUserName, "\")
    sessionHandler.UserName = ...