How to Logout in Windows Authentication?

I am using Windows Authentication for my project. If Admin log out the control has to go to main menu page. In Main Menu page Icons are there for Admin and Employee. When Admin or employee clicks that icon,  immediately authentication is required? But now only if the brower cache are cleared authentication dialog appears but I need it to display authentication dialog whenever they are going from Main menu page.
Is there an ability to logout and login as a different user needs to be available as some users share their PCs, without doing windows logoff. Since the credential are stored in Server DB using roles.
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