Ok, here's the deal. Whenever me and my friend try to connect our computers over Remote Assistance there appears some strange error. We both have the newest version of Live Messenger.
This is what the error says when my friend tries to send an e-mail invitation over Remote Assistance:

Your invitation was not sent because you need the latest version of Messenger to use the Remote Assistance feature. Please go to the Windows Live Messenger update site to install the latest version

My friend already tried to re-install the Messenger but the error still appears. Maybe someone have some ideas what the problem might be?

go to control panell->classic view->administrative tools->services
and check if "remote acces connection manager" and "remote desktop help session manager" arnt disabled

Thanks man, we'll see today if that works :D