Using MSN without installation on your pc
It is feasible to use an MSN client for discussing with your friend without installing the software. There are some MSN clients messenger created to be used on web pages allowing you to get in touch with your msn contacts.
Moreover their features are very limited as there are no audio and video files exchanges.
The official Microsoft site (Web Messenger):
http://webmessenger.msn.com/
Other alternatives:
E buddy also support MSN, Yahoo, AIM, GoogleTalk, Facebook, ICQ and MySpace clients :
Meebo is a web messenger allowing you to access your accounts anywhere and can support msn, icq, yahoo, gtalk, jabber.
Adobe flash wont install for msn