Francis,
It may be an issue with NTLM. See the article here:
http://technet.microsoft.com/en-us/library/bb727037.aspx, down to the bottom:
Cannot Authenticate to a Shared Folder from a Windows Vista-based Computer
If you cannot authenticate when accessing a shared folder from a Windows
Vista-based computer on a computer running a version of Windows prior to
Windows XP (such as Windows 98 or Windows 95), a computer running an
operating system other than Microsoft Windows, or to a network device, the
cause might be a mismatch in the configured support for NTLM 2, an
authentication protocol that is used for file and printer sharing
connections. By default, Windows Vista is configured to use NTLM 2.
To resolve this issue, you can do one of the following:
Enable NTLM 2 support on the computer or device to which the Windows
Vista-based computer is attempting to connect. For computers running versions
of Windows prior to Windows XP, see How to enable NTLM 2 authentication. For
computers running operating systems other than Windows, see the operating
system's product documentation for information about how to enable NTLM 2
support. For network devices, see the device's product documentation or Web
site for information about how to enable NTLM 2 support or download a
firmware update that supports NTLM 2. This is the preferred solution.
If you cannot update the computers running operating systems other than
Windows or your network devices to support NTLM 2, change the
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa\LMCompatibilityLevel
registry value on the computer running Windows Vista to 1. This solution is
not recommended.
