By default WinINet supports the following authentication schemes: Negotiate, NTLM, Digest and Basic. However, in previous OS releases WinINet did not support Negotiate authentication to proxys, as it would not respond to a negotiate challenge from a proxy server (See http://support.microsoft.com/?kbid=321728).
We have added support for negotiate authentication to a proxy server in Windows Vista. KB article 321728 is now obsolete and will be updated for the Vista Beta 2 release.
For more information on WinINet authentication, follow the link below: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wininet/wininet/handling_authentication.asp .