I suspect that File and Printer Sharing for Microsoft Networks has been disabled on the remote computer.
On the remote computer:
1. Right-click My Network Places and press Properties.
2. Right-click the LAN connection and press Properties.
3. Check the File and Printer Sharing for Microsoft Networks box.
4. Press OK.
The above action registers the computername with a NetBIOS suffix of <20>, which is required by most remote management tools.