Consider the following scenario.
In this scenario, there is a prompt to enter credentials even though the user account that being used has sufficient permission to access this site.
For example, when opening a Microsoft Office file from a Microsoft Office SharePoint site by using 2007 Microsoft Office on a Windows Vista-based client computer that has no proxy configured, the user is prompted for authentication.
The following error may also be seen when working with moved folders via explorer view:
"Your client does not support opening this list with Windows Explorer."
Note: This problem does not occur on a Windows xp-based computer.
Important This hotfix is included in Windows Vista Service Pack 1 or a later service pack. However, the AuthForwardServerList registry entry must still be configured. For more information, see the Registry information section.
This issue is not related to the Proxy.
In Windows Vista, Internet Explorer uses the WebClient Service when using Internet Explorer to access a WebDAV resource. The WebClient Service uses Windows HTTP Services (WinHTTP) to perform the network I/O to the remote host. WinHTTP sends user credentials only in response to requests that occur on a Local intranet site. However, WinHTTP does not check the security zone settings in Internet Explorer to determine whether a website is in a zone that lets credentials be sent automatically.
If no proxy is configured, WinHTTP sends credentials only to Local intranet sites.
To fix the issue, create the registry item below in Windows:
After this registry entry is created, the WebClient service will read the entry value. If the client computer attempts to access a URL that matches any of the expressions in the list, the user credential will be sent successfully to authenticate the user, even if no proxy is configured.
Note: The WebClient service has to be restarted after the registry is modified.
There are no prerequisites for installing this hotfix.
The computer has to be restarted after applying this.
For more details, please refer to http://support.microsoft.com/kb/943280