I tested connecting to your site on Windows Vista and it does work for me. It is likely a configuration issue on your computer. Try the following:
Inside of Internet Explorer, click on the Tools menu and select Internet options. From there, select the Security tab. Click on [Custom Level] and look for User Authentication. Underneath this, ensure that Automatical Login only in Intranet zone is selected.
If this does not help, please see the following site: http://www.webdavsystem.com/server/documentation/troubleshooting
Poetic Apricot (Matthieu)
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