This is indeed what the problem was (thanks to John and David). 
Neither the mobo manual nor the CD labelling mentions this in any
significant way (it's kind of buried in the manual) but it's among the
first things the system installs from the CD.

It's been years since I've built a system, and I'm not used to having
all this stuff on the mobo.

Mark Shaw

On 4/27/06, John Sproule <[log in to unmask]> wrote:
> I would recommend installing the nvidia chipset drivers off of the CD, you
> can update them later if you want to.  I'm pretty sure that this will give
> you the drivers you need for the NIC controlled through the nvidia
> motherboard chipset (actually chip).  Your motherboard may have another
> ethernet port (many higher end boards do these days).  As Mark Russ
> suggested, this will require a different driver, such as one from Marvell.
>
> This should give you access to your network connection(s); so, you can
> download updated drivers and windows updates.
>
> John Sproule
>
> ----- Shortened Original Message -----
> From: "Mark Shaw" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, April 27, 2006 6:24 PM
> Subject: [PCBUILD] Asus A8N-SLI and onboard ethernet
>
>
> I'm buiilding a new system: Asus A8N-SLI  (socket 939), Athlon64
> (3700MHz+), and a fresh install of XP Professional 64.
>
> XP does not see the onboard ethernet.
>
> Note that when I installed XP the ethernet port was disabled. I have
> since enabled it in the BIOS and rebooted. Also I have not yet
> installed anything from the ASUS CD -
>
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