Windows XP comes with the Nvidia drivers.
They don't work on 2K. I had a GeForce2 MX on this 2K box, fresh install of 2K, and it went nuts. I always put the Detonator drivers on any box with an Nvidia card (and I have 2 XP boxes with family and one personal 2K box).
What looks like what happened was that Windows "forgot" the drivers was there. I've seen it happen under 98 and 95. Since TCP/IP gets bound to the card, it goes nuts and takes TCP/IP with it if the driver goes. I reproduced this on NT4 a while back also with a PCMCIA network card (note: NEVER use NT4 with a notebook if you can avoid it! When you remove it, you also remove TCP/IP and the ability to even bind to a local interface!). Going into the registry and screwing around has fixed it for me, and it involves getting the PNP string, searching for it, and deleting all relevant occurences under HKEY_LOCAL_MACHINE and the other hives.
DVD-ROM drives do this under Windows too, especially when you install Adaptec software.
Do you have a 3Com card? That's one of the cards that did this. The other was some no-name we had 5 years back, when I ran into this issue under Windows 95. In that case, reinstalling the driver worked.
The other thing I've seen happen is that sometimes NT/2K's routing tables get completely messed up. I've reproduced this under 2K and NT. What it'll do is bind the default route to the wrong interface. Pretty wacky. It does that with multiple interfaces, mostly. However, you can mess with those without rebooting
. route -p under Windows is your friend sometimes, since Windows stores routing info with the drivers in the registry, apparently. No text files for that, that I saw.
If you're multihomed, especially, you'll need to bind the routes to your primary DNS servers to the internal interface. Especially if you block traffic on the external interface! This is because any name resolutions on the external interface get borked trying to get them on the secondary interface. NT4 and 2K fall victim to this one.
The best thing to do, regardless, is get 2K on there. Upgrades have "issues" with a lot of DLLs that applications put in for Windows 9x that don't work right under 2K. XP fixes this pretty well, since it's got a better 9x emulation layer. However, 2K has a real TCP/IP stack, which 9x/me does not.
Mitch