I had this same issue with a website running on IIS7.0 Windows Vista. I ran the "netstat -ano" command and found out that something was listening on port 80. Using the "tasklist /svc" command, I checked the PID and found out that the Skype service was listening on that port. I stopped it, started the website manually and started the Skype service again. That resolved the problem.
This happened when port 80 was used by Citrix ctxxmlss.exe. Changing the ctx port via script "ctxxmlss /R088" at first was impossible because one of the svchost.exe processes used that port, which in fact prevented at install-time to set port 88 for ctx (“this port is not available”).
Killing svchost on port 88 and attaching port 88 to ctx made it possible to start IIS on port 80.
I had this issue when trying to open up our Citrix site on the server. I checked the web site properties, port 80 and port 443. The certificate was installed but "Require SSL" was not checked. The URL that was being used was HTTP and no redirection was specified for HTTPS. I ran "netstat -ano" and something was listening on 443 (PID 2056). I opened Task Manager, checked this PID number and SSLServerRelay was on this port. This is a service from Citrix. I stopped this service and set it to start manually.
I had this issue on a Small Business Server 2003 Premium Edition system. It turned out that ISA 2004 was intercepting all HTTP requests and thus, causing problems with IIS. I had to disable the Web Proxy Filter in ISA. See the link to "Troubleshooting Web Proxy Traffic in ISA Server 2004" for information on how to do that.
for details on fixing this problem.
Resolved this problem by setting the port that was being used by my HP Power Manager from 80 to 8080. Seems it was blocking registration of port 80 (or 443 if you use SSL) by IIS.
This may occur if another application is using port 80 (i.e. another web server application).
Some newsgroup posts suggests that this may occur if the computer does not have a valid IP address.