Using Logon Form with Windows Authentication

I have created an Intranet site using windows authentication based on role membership.  It is working great.  My Web.config is shown below.
<!-- AUTHENTICATION & AUTHORIZATIONS -->
   
  <authentication mode="Windows" />
  <roleManager enabled="true" defaultProvider="AspNetWindowsTokenRoleProvider" />
  <authorization>
   <allow roles="D820\Developers, D820\IT, D820\Staff, D820\Providers, D820\Supervisors, D820\Directors, D820\Interns" />
      <deny users="?"/>
  </authorization> 
Now my user wants the capability of being able to login to the intranet site as another user.  For example, IT guy needs to login on a user's system to set some data.
I have created a standard MS Login Page.  However when I t ...