Onboard network lost just after login

We tried setting up a new connection because the computer owner said this had happened before and he got the network back. PC asked to reboot and we did.

Just after the computer logged in, we saw the system tray icon come up saying that some of the network drives were unavailable. I double-clicked it to bring it up and check that it was only talking about the ones at his home. (He's currently at a LAN doing nothing much!) The icon immediately disappeared and we found that under his network connections, he again had no adaptor showing up for his ethernet adaptor.

The only thing that consistently shows up is his Hamachi connection - this shows up in Network connections, ipconfig via DOS window and in Device Manager. The onboard network adaptor only shows up under Hidden Devices in Device Manager.

We are logged in via Administrator in XP (Pro I think). AVG 8.0 Free, don't know about other security software - possibly not much because his dad runs his own PC business and has them behind a lot of protection when the computer is at home. I'd ask Dad to take a look but Dad is asleep while his son, mine and others waste the night hours gaming! (Or fail to in the case of four of them with network troubles. ;-) )

I'm taking back the absolute latest in AVG updates and we'll scan and I may try uninstalling and reinstalling in Safe Mode or using a System Restore Point but I'm hoping someone may be around who can take a quick look and offer some pointers. I've never come across anything like this before and haven't found anything that sounds likely in my search.

Unfortunately we don't have the internet there so I'll have to hope someone answers before I leave here or SMS them some suggestions when I get back if we haven't resolved it.

Thanks for any suggestions!

EDIT: Forgot - we can ping 127.0.0.1 and the computer by its' name and the Hamachi connection IP. We can't ping by any other expected number; can't ping anyone else; no lights come up when we connect him to the switch; and on his computer when we plug in the network cable, only the orange light comes up (I think the other is green). Neither does ipconfig bring up anything but the Hamachi adaptor.

 

Make sure the device is connected. Try to use a network cable and a port on the router that you know work.

Run Device Manager (Start --> Run --> "devmgmt.msc"). Under Network Adapters, find the one that corresponds to your network card. Right-click it and choose properties. On the Power Management tab, uncheck "Allow the computer to turn this device off to save power" and then click OK. Also, make sure the device is enabled. If there's a red mark on the icon in Device Manager, the device is disabled. Reboot the computer.

Next, go Start --> Control Panel --> Network Connections. Then go View --> Details. Make sure the Network Connection that corresponds to the network adapter is not listed as "Disabled", and is not listed as "Network cable unplugged". If it's listed as "Network cable unplugged" check the cable again. You're not getting basic electrical connectivity through the wire, which means the cable is damaged, the computer is damaged, the router is damaged, or the OS is very very screwed up.

Now open a command prompt and run "ipconfig /all".