Check using netstat -aon or netstat -aon | findstr 0.0:80 in a command prompt to see which Process Id is LISTENING to port :80 and then watch for that Process Id (PID) in Task Manager with view->select columns-> process id checked. End that process, restart IIS and you are done. (Note: if you have Skype installed, try exiting that process first.)
In a modern Task Manager, you need to go on the Details tab to search for the PID. Or, as mentioned by @Nikita G in the comments, you can use this command to find the task from your command prompt:
tasklist /FI "PID eq 123"
Note: change 123 with the PID returned from the first command.