As Steven posted earlier, if you try to display a password-protected page in SuperPreview, the program instead displays the page that appears when a site visitor tries to access the page without supplying login credentials.
I wanted to follow up Steve’s post to briefly clarify that SuperPreview does support NTLM authentication, which is commonly used by websites on corporate intranets. Pages on networks that use NTLM authentication don’t require the user to login to view the pages, instead, they rely on the site visitor’s Windows login credentials. So if your computer is on the same network as the page that relies on NTLM authentication, you should have no problems opening the page in SuperPreview. (For in depth information about NTLM, see http://msdn.microsoft.com/en-us/library/aa378749(VS.85).aspx.)