i have this VERY bizarre problem and have scoured the internet to find a solution but no luck. i have just built out 9 servers with Win 2K3 Ent.
here is what it isn't - a firewall issue, a clock issue on the bios and the OS, an internet issue
this is what i am able to do - i can go to Microsoft update via IP address only. any website that has Microsoft.com does not display but EVERY other website in the world works.