How DNS queries are forwared in a Windows Workgroup network

view story

http://serverfault.com – Situation: 20 computers in the same windows workgroup. They are connected with some switches. Then there is router and a firewall (2 separate hardware, but I don't know how they are connected) with the physical ADSL modem to the internet. Computers have mostly windows XP, some windows 7. In the TCP/IP configuration of each computer, there is a fixed IP address, and the gateway is the router's IP. The DNS are those of the ISP, or the IP address of the router. In the router configuration (via http), I saw a page where DNS was set to "from ISP". Suddendly happened a problem: when browsing websi (HowTos)