You cannot open a trusted WebDAV folder by using the SharePoint Explorer View feature in Windows Vista or in Windows Server 2008
Assume that you have a Web Distributed Authoring and Versioning (WebDAV) folder. The folder is a trusted site in Windows Internet Explorer on a computer that is running Windows Vista or Windows Server 2008. However, if you set the Autologon policy security level to medium, you cannot open the WebDAV folder by using the Microsoft SharePoint Explorer View feature. Additionally, you may receive an “access denied” error.
Note: You may also receive other error messages, such as the following error message:
Your client does not support opening this list with Windows Explorer.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB2577797.