I am very puzzled and without a clue as why one of my PCs will fail ("A network cable is unplugged") to connect to my LAN using a particular outlet, but if I connect another PC, as a notebook, on the same outlet there are no LAN connection failures. Yet if I connect the desktop to another outlet, it connects to the LAN without a problem.

Details: Desktop Dell Dimension 8200, running all latest versions of software (updated nigthly) and, apparently, virus free (scanned nigthly.) I have Windows XP Pro 2002 SP3, AVG Free, Windows Firewall on, Winpatrol, and have a Realtek RTL8139/810x Family Fast Ethernet NIC NIC with latest driver (25/03/09.)
For the wired LAN I use a Linksys WRT610N V1; I have several Ethernet outlets throughout the apartment.

Of the Dell Dimension 8200, I have changed the NIC, and I have also changed the cables (cat 5E) PC to wall and have ping the router to Ethernet outlet cable connection (OK.)

When I plug the Dell to the wall outlet, it begins to acquire the IP address (sometimes it gets it momentarily or for a short while and connects to the LAN and to Internet) and then, right a way or moments later, I get the message "A network cable is unplugged".
If I connect a notebook (wireless connection exclusively) to the same outlet, it connects to the LAN and Internet without problem.
If I take a long cat 5E cable and connect the Dell to another of the wired LAN outlets nearby, it connects to the LAN/Internet without a problem. I have also tried the Dell, with success, to connect it to the LAN via a wireless adapter.

Why the Dell desktop fails to connect on a particular Ethernet outlet?

Maybe it's a cable? Try swapping?