What the OP probably means is, is it possible to talk to people on Google Talk in Windows Live Messenger? Which, the answer would be no.
- Windows Live Messenger -> Google Talk
Microsoft and Yahoo! made an agreement in 2005 to allow users to talk across their IM platforms, that's the only reason that's possible.
- Windows Live Messenger -> Yahoo! Messenger
However like djdannyp said, a Gmail user can sign up with their email address and use Windows Live Messenger. It's not restricted to Hotmail only.
- Windows Live Messenger -> Windows Live Messenger