If you run a web server on your machine and you have Skype, you might have problems with Skype preventing your web sites from starting. If you try to start them you'll get a message stating that the resource is in use... this is because Skype automatically takes over ports 80 and 443 as "alternatives for incoming connections."
In case you don't know, port 80 is the default port for http connections, and port 443 is the default for https connections. I guess this might be fine for home users but for developers, having those ports automatically taken over is horrible. To fix this problem start up Skype and go to Tools > Options > Advanced > Connection and uncheck the box next to "Use port 80 and 443 as alternatives for incoming connections."
You should now be able to start your websites.