This article explains why you may not be able to map a network drive to a Document Library in Widows SharePoint Services 3.0 or 2.0.
If you are connecting using Windows Shell Components, such as using the net use command, You may receive the error message: "System error 5 has occurred." or "Access is denied."
If you are connecting using Explorer View you may be prompted for your username and password several times. If you click Cancel Web folders are displayed.
This occurs because by default Windows Vista and Windows XP SP2 disables Basic Authentication in Webdav for security purposes.
Below is the support article from Microsoft that discusses this error and offers a workaround:
Note: The workaround does have a disclaimer about modifying your registry, which may cause problems if the directions are not followed correctly. We recommend backing up your registry before attmepting this workaround.