Added computers can’t access internet

I’ll try to make this as simple as I can ..

I have a Windows 2000 server running both DHCP and DNS for client computers (mix of Win2000 and XP Pro), and I think I’m missing something big here. Until yesterday everything was fine, but now whenever I “add” a computer to the network (not to the domain, just plug it in, such as a laptop) it can see local network resources such as printers, server, etc.., but cannot see the internet. The result is the same across 2 “test” laptops and 1 desktop so far – which is why I believe I could have a server/firewall issue.

Is there anything on my server that could be blocking the “new” computers to the network from accessing the internet?

Thanks!