Unable to start web site IIS HRESULT: 0x80070020)

In my case Skype was the culprit. Yes, you read it correctly: Skype was using port 80/443.

Below is the way to disable it-

1. launch Skype -> Tools -> Options
2. go to advance settings -> connection

3. uncheck the "use port 80 and 443 ...."

   

4. save and quit your Skype or restart your machine.

To further elaborate Nacho's (correct) answer, from here:

Run the following command from a command prompt to find the PID of the process which is using TCP port 80 and/or 443 (hint: for 443, it's probably Skype):

netstat -aon | find ":80" 
netstat -aon | find ":443"

You will see an output similar to the following. Remember the actual PID will vary from case to case.

TCP    0.0.0.0:80            0.0.0.0:0              LISTENING       3604 
TCP    0.0.0.0:443           0.0.0.0:0              LISTENING       3320

Now using Task Manager you can easily find out to which process the above PID belongs and take appropriate action.

Check this http://support.microsoft.com/kb/890015/en-us

Another process is using port 80 or port 443 on the computer that is running IIS. By default, IIS uses port 80 as the default TCP port and port 443 for Secure Sockets Layer (SSL).