There seem to be some problems with the WebDAV (webfolders) implementation of Windows Vista.

This patch from Microsoft should fix the issue (or most of the issues...):

http://www.microsoft.com/downloads/details.aspx?FamilyId=17C36612-632E-4C04-9382-987622ED1D64&displaylang=en

Also consider using WebDrive or NetDrive instead.