Is it onboard ethernet or a card ? ?
Either way buy a new card and install it...around 15 bucks.
If this produces no joy joy then try another ethernet cable....it's one or the other.
I'll second this 100% I had a card actually I bought two of them at the same time for two different computers.. The one worked fine for years and one day I started up my computer and no network! Same thing though, device manager said it was fine, but it was dead... I couldn't figure it out. Stuck another one in there and it worked great. It simply died. It happens. It was a Network Everywhere card, pci... And btw, I decided to fix up that other computer and use it a few days ago and guess what---NO NETWORK... and it's the other card that I had bought when I bought the other one that died. So I'm thinking it crapped out as well. But yeah try another card they're cheap, esp if you look online, you can get one for like $5 on ebay, haha.
Also off topic, and not the cause obviously, but 256mb of ram for xp (esp pro) is pretty bad, I would definitely consider upgrading that to 512, it should be almost twice as fast then. Just a suggestion!
Jay