Port 80 is being used by SYSTEM (PID 4), what is that?

A new service called "Web Deployment Agent Service" (MsDepSvc) can also trigger "System" with PID=4 to listen on port 80.

There are many services, which can listen port 80 on windows.

Luckily you can detect and stop them all running simple console command:

NET stop HTTP

When you'll start it, you will get list first:

To avoid this problem in future go to Local Services and disable listed services.

N.B. - Some services will restart themselves immediately, just run 'NET stop HTTP' few times.

Also, try stopping "SQL Server Reporting Services (MSSQLSERVER)", that apparently defaults to 80. I did that and port 80 freed up. PID identified the culprit as "System", but apparently that System can mean multiple things.

I had the same problem. Could fix it by stopping the World Wide Web Publishing Service under running services.