VS 2008 - Logon Failed Due To Server Configuration

I am using Integrated Windows authentication and Role based authorization.The web.config file has two Active Directory groups.Sample code of web.config file for Role based authorization (Actually I got this sample code from a moderator of this site Gary works well).

Code:
<location path="Bloodgroup.aspx">
<system.web>
<authorization>
<allow roles="MedcenterGroup1"/>
<deny users="*"/>
</authorization>
</system.web>
</location>

So user's who doesn't belog to "Group1" AD group, should get Unauthorized error. It works exactly like that. But, before getting unauthorized error, it shows a pop up screen (as shown in the attached picture) prompting to put userid and password. If a user in AD group "Group1" accesses "Blood group. aspx" page, popup message doesn't come which is correct. Following is the code in web.config file to handle errors:

Code:
The <customErrors> section enables configuration of what to do if/when an unhandled error occurs during the execution of a request. Specifically, it enables developers to configure html error pages to be displayed in place of a error stack trace. -->

<customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
<error statusCode="401" redirect="Errors.aspx" />
<error statusCode="403" redirect="NoAccess.htm" />
<error statusCode="404" redirect="FileNotFound.htm" />
</customErrors>

Question 1: why does the popup box prompting user id and password comes up?
Question 2: I have put in web.config file in customErrors tag to direct to Errors.apsx page when I get 401.2, it should be redirected to Errors.aspx page. But it is not getting re-directed.