EADDRINUSE means that the port number which listen() tries to bind the server to is already in use.
So, in your case, there must be running a server on port 80 already.
If you have another webserver running on this port you have to put node.js behind that server and proxy it through it.
You should check for the listening event like this, to see if the server is really listening:
console.log('ok, server is running');
Hope it helps!!