I enabled IIS 7 on one of my Windows 7 box. When I click start the default site, it always give me error. Said something wrong of the setting. Only when I change the binding port to other than 80, it works.
So, I think port 80 is occupied by other program.
I google it and find the way to point it out.
1) Command line.
So it is clear that Skype use Port 80