Vista cannot access Internet through router

Hello,

I have a home network that has one XP machine set up as an internet gateway, shared by multiple computers behind a router. Two of the computers behind the router are Vista, and one is XP. The XP computer can go through the router and reach the internet. The Vista computers can not reach any external IP address, but they can ping the gateway computer and the router.

I have DNS set up on the gateway computer. The message I get back when I try to reach the internet from the Vista machines is "router IP address: destination net unreachable", and it is using the IP address of the web site I am trying to reach, so DNS appears to be working. The only difference I see between the XP setup and the Vista setup is that the XP machine has the internet gateway set to be the gateway computer's IP address, and the Vista machines have it set up to be the router's IP address. I tried changing the Vista computers manually to have the gateway computer's IP address as the internet gateway, but ended up with two separate networks and many warning messages from Vista that I should disconnect one of the networks. I do not quite understand why this is happening.

I should mention that when I first connected the Vista computers, they found the internet just fine, so the router was able to handle IPV6 or whatever Vista uses. Then we changed internet providers and now I have these problems. By changing internet providers, I have a new modem, and may be using a different ethernet card (can't remember if I changed it or not).

Does anyone have any suggestions?

Thanks!