Hi, thanks for the quick response again. I have found the trick to make that popup dissapear, I want to share what I did and worked for me in case that anyone else have the problem.
Just have to configure the redirecting on Error Pages as following:
1.- Open IIS, expand the directory tree till you reach "Alias"/DesktopModules/AuthenticationServices/ActiveDirectory/.
2.- Double click "Error Pages" in content pane, you will be redirected "Error Pages" page.
3.- In this page, right click "401" and select "Edit".
4.- Select "Response with 302 Redirect" and Enter your DotNetNuke web portal's address so that it will be redirected back to form authentication when window authentication fail to process.
And thats it!! the popup went away and is not bothering me anymore. Now I can select between Standard and Windows Login in the portal's login page.
Thanks for your time, patience, support and everything! See ya.