What does the error.log show as the cause?
First place I'd check is to see if anything else is listening on port 80 (another webserver such as IIS or a different instance of apache; skype, etc). If there is, your options are either to map the apache to a different port than 80 (8088 is a common choice as its user available ports), and then either attach via 8088 or add 8088 to the services file for http traffic, or shut down the program using the port.
The error log should tell you what the actual cause is.
header('HTTP/1.1 420 Enhance Your Calm');