This occurred when I configured a machine to run both IIS and Windows Media on the same box. I wanted WM to be able to use HTTP as well. In order for both services to use port 80, I added an additional IP to the box, then ran the utility in systemroot/windows/system32/windows media/server/admin/mmc/WMSHttpSysCfg.exe. This allows you to specify the IP(s) that the WEB server will use for port 80.
From a newsgorup post: "You will need to use the HTTPCFG utility included with the Windows Server 2003 Support Tools. These tools are located on CD #2. From a command prompt, type the following command: “httpcfg query iplisten”. You should see your internal IP address and 127.0.0.1 (localhost). Next, type the following command: “httpcfg set iplisten -i x.x.x.x”, where "x.x.x.x" is your external IP address. Again, perform the “httpcfg query iplisten” command to view the results. At this point, attempt to start the website. If it still fails, it may be necessary to restart the server for the changes to take effect".