How to fix Error: listen EADDRINUSE while using NodeJS?

If I run a server with the port 80, and I try to use XMLHttpRequest I am getting this error: Error: listen EADDRINUSE

Why is it problem for NodeJS, if I want to do a request, while I run a server on the port 80? For the webbrowsers it is not a problem: I can surf on the internet, while the server is running.

The server is:

  net.createServer(function (socket) {
    socket.name = socket.remoteAddress + ":" + socket.remotePort;
    console.log('connection request from: ' + socket.remoteAddress);
    socket.destroy();
  }).listen(options.port);

And the request:

var xhr = new XMLHttpRequest();

xhr.onreadystatechange = function() {
    sys.puts("State: " + this.readyState);

    if (this.readyState == 4) {
        sys.puts("Complete.\nBody length: " + this.responseText.length);
        sys.puts("Body:\n" + this.responseText);
    }
};

xhr.open("GET", "http://mywebsite.com");
xhr.send();