How do i sign on to windows live messenger with a yahoo contact, to message contacts on msn?

You can invite your Windows Live? Messenger contacts to IM with you without joining Yahoo! Messenger.

Windows Live? Messenger and Yahoo! Messenger contacts can IM without having to switch to a different Messenger application!

Inviting Friends to Use Messenger (Tutorial)

http://help.yahoo.com/us/tutorials/ms8/m...