Problem accessing domain computers from Active Directory Server

Hi all..

I have a small network which is controlled by a Windows Server 2003 machine.
This machine acts as a domain controller, DNS and Exchange Server.

When I open Active Directory MMC and go to computers node, I can see all
domain computers which are running Windows XP (and which were added to the
domain).

When I click Manage option in the popup menu when a computer is selected,
Active directory cannot find network path sending an error message box
telling me that. All computers were added to the domain using Mi Computer ->
Properties -> Network ID -> Properties

Another curious fact is when I try to use NET SEND command, it responds with
the error that message alias isn't found on the network.

Any help to solve this will be greatly appreciated
Thanks

Jaime