Show or Hide the login window when accessing a site that uses Windows Authentication.

You're working on a site that is set up for Windows Authentication.  Two scenarios:

1. You want your credentials to "pass through" without logging in.  In IE, open Internet Options, Security tab.  Select the zone the site is in, or add it to a zone if appropriate.  Click the Custom Level button.  Scroll down and select the "Automatic logon with current username and password" under the User Authentication, Logon section.
2. You don't want to be logged in automatically so you can log in as other people for testing or whatever.  Follow the steps above except choose the "Prompt for username and password" radio button.