I got the following error message when trying to start a site on port 80 in IIS

Internet Information Services (IIS) Manager

The process cannot access the file because it is being used by
another process. (Exception from HRESULT: 0x80070020)

Following the instructions here, I:

1. Ran netstat -aon | find ":80" from a command prompt
2. Found the PID for the process using port 80
3. Used Task Manager to find out what program owned this process. It turned out to be Skype.
4. Closed Skype. Lo, I can now start the site in IIS