SBS 2003 cannot connect via port 80

Hi,

I have a problem with SBS 2003. My network is setup with all traffic going to SBS for DNS resolution, then out through a common gateway. All computers within the network, except for the SBS server, can view webpages and access information through port 80, but SBS for some reason beyond my understanding. There are no firewalls preventing port 80 access, the server can be accessed from within and without via port 80. Does anyone have any information that may help me track this problem down?