We're using IE as out browser and I checked and the site is indeed in the "Trusted Zone". I made the changes to the web.config you suggested. Now, when my previously logged out user follows the link

http://<DotNetNuke_Install>/DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx

they still get the Windows log in prompt. Entering the username and password takes them to the Home page with minimum permissions and does not show them as being logged in.

I can now log out a previously logged in user but then going to the login page still does not give them option of using a "Windows Signin". It's DNN Forms only.

I'm still conecerned that I can't configure Active Directory Authentication on the Extensions page. I get the same error I received previously. I considered removing the AD Authentication Extension and then reloading it but can't see how that is done.

Yes, the same error as previously. I didn't change any web.config lines.

I think I'm going backwards... I just tried uninstalling the DNN ActiveDirectory extensions using the "Host/Extensions" page and reinstalled version 5.02. Now I can't bring up the site and get the following error:

Server Error in '/' Application.
--------------------------------------------------------------------------------

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. 

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
   DotNetNuke.UI.Skins.SkinController.GetDefaultPortalSkin() +19
   DotNetNuke.Entities.Host.Host.get_DefaultPortalSkin() +59
   DotNetNuke.Entities.Portals.PortalSettings.get_DefaultPortalSkin() +19
   DotNetNuke.Entities.Portals.PortalSettings.GetPortalSettings(Int32 tabID, PortalInfo portal) +1369
   DotNetNuke.Entities.Portals.PortalSettings..ctor(Int32 tabID, PortalAliasInfo objPortalAliasInfo) +113
   DotNetNuke.Common.Globals.GetHostPortalSettings() +116
   DotNetNuke.Common.Globals.GetPortalSettings() +83
   DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule.Init(HttpApplication application) +234
   System.Web.HttpApplication.InitModulesCommon() +66
   System.Web.HttpApplication.InitModules() +46
   System.Web.HttpApplication.InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) +700
   System.Web.HttpApplicationFactory.GetNormalApplicationInstance(HttpContext context) +281
   System.Web.HttpApplicationFactory.GetApplicationInstance(HttpContext context) +118
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +290

I tired clearing the browser cache and cookies an restarted the server to no avail...