Socket.IO Server Runtime Error/Exception

I have this code in a file named server.js: 

var app = require('express')();
var server = require('http').createServer(app);
var io = require('socket.io')(server);

io.on('connection', function(){
    console.log("a client connected");  
});

server.listen(80);

When i run node server.js in the terminal (cmd), it throws this error:

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: listen EACCES
    at errnoException (net.js:904:11)
    at Server._listen2 (net.js:1023:19)
    at listen (net.js:1064:10)
    at Server.listen (net.js:1138:5)
    at Object.<anonymous> (C:\inetpub\wwwroot\socketio\server.js:9:8)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)

Do you have an idea of what could be the problem? Thanks!
Note: I already have socket.io and express installed.

Looks like the port 80, is invalid or unacceptable. I tried to change it to another port (8000) and it worked. 

server.listen(8000);

The exception is gone. I hope I can find a way to distinguish between valid/invalid ports at run-time.

It seems you are not allowed to open a serversocket on port 80. See this line in the error message

C:\inetpub\wwwroot\socketio\server.js:9:8

Socket addresses smaller than 1024 are reserved for system use. Try this:

var app = require('express')();
var server = require('http').createServer(app);
var io = require('socket.io')(server);

io.on('connection', function(){
    console.log("a client connected");  
});

server.listen(3000);