as maurito said, skype could be the problem, and athor programs that listen on port 80. So, by a netsetat -ao you can see all connections on local machie, and you can identify by PID the process that is listening on port 80.
If skype is the one, there is an option in connection tab in skype that says
Use port 80 and 443 as alternatives for incoming connections ...uncheck it
