Urgent !!!! WAN not really responding

Hi all,
we got the problem, that we can’t get any webpages, if we type in the name i.e. “www.msn.de”. If we typoe in the IP-Address, i.e. “207.46.28.116” we get the page.
Any ideas??????

Our Network-configuration as follows:
1 PDC, 1 BDC, 2 DNS-Server (1 on each), 2 WINS-Server (1 on each), 1 DHCP-Server (on the PDC).

Regards
Thomas