Hakimi … First you need to know if you can ping the server by IP address. If you can ping by IP but not by name then you have a DNS issue. Here is something very important:
You should never point your internal machines to an external DNS server through your ISP. You should actually install and run DNS on your 2k3 server and configure the DNS address of the ISP as a forwarder for your domain.
Internal machines will get their DNS queries resolved my your server (much faster than waiting to fetch it from an ISP) and your server will only go to the internet for records that it does not already have cached.
Look at this Microsoft link: