ADSL with Active directory

The basic problem is you're not getting a valid IP from the DSL modem. Have you verified connectivity to the DHCP source? Where was the server getting it's IP lease from before joining the domain? Has the placement of the server changed? Like beforejoining the domain, it was outside a firewall? Did the firewall do DHCP?
Did the server browse through the Proxy server before joining the domain?

You didn't mention if you want this server to browse the internet through the proxy server or ifit was ok before joining the domain. If the only access to the internet is through the proxy server, you have to install the proxy client. Using Windows Explorer, browse to the Proxy server's mspclnt share and run the proxy client's setup.exe file. After you reboot, check IE's connection settings to ensure it is configured to use the Proxy server if you use the Webproxy service or cleared if you use the Winsock service.