Just create a shortcut on the desktop for gtalk. If you have an issue creating the shortcut, navigate via the start menu to gtalk , right click -> copy -> right click on desktop -> paste.
Now right click the shortcut, and chose 'Porperties'.
In Target, add a /nomutex option at the end. E.g. : following is how it looks like in my shortcut.
"C:\Program Files\Google\Google Talk\googletalk.exe" /nomutex
Now, just click on the gtalk shortcut to start a new instance of the talk messenger. Add your gmail and domain accounts.
Have fun !