Lights show on Network Card until you turn the PC on.

The Problem
PC with Gigabit NIC. NIC shows lights when powered off, indicating that the wiring is correctly connected. However, as soon as the power is switched on to the PC, those lights disappear.

The PC
hp Compaq d530 USDT

The Solution
Although the PC has a Gigabit NIC, and was plugged into a gigabit switch, I tried to plug it into one of the remaining 10/100 switches (I only have one left!), and this actually fixed the problem.

It is likely that a new motherboard (since it's an on-board NIC) would also have solved the problem, but since the machine was out of warranty, I wasn't planning on spending that kind of money on it!

As it happened, a couple of months after I managed to solve this problem, I found another PC exhibiting the same symptoms. Thankfully I'd emailed someone about the solution, and was able to find out what I'd done the first time around.

The Back Story
I don't know if you've come across anything like this before, but it caused me some months of misery before I worked out the solution.

The lights were lit on the network card when the power was off - which usually indicates to me that there is, in fact, a connection, but as soon as the power was turned on, the lights went off. For ages I was convinced that the wiring was to blame, and that the socket was labelled wrongly. Since a couple of other sockets had been wrongly labelled in the Great Re-Wiring of November 2007, I assumed that this was the case, and spent quite a lot of time checking nearby connections (there were two other wrongly labelled pairs, but both were next to each other) in the hope of finding the correct connection. Sadly, the user would turn the PC on every morning in the hope of finding that it was miraculously fixed, and thus there would be no evidence of connection at the switch or on the PC. The time taken to work out that the connection was, in fact, valid, was due to this user-based optimism.