Right Click on the Google Talk icon (in the start menu, either in your quick launch or, Start > All Programs > Google Talk > Google Talk), then click Properties, then add /nomutex at the end of the line, for example, in my setup this looks like:
“C:\Program Files\Google\Google Talk\googletalk.exe” /startmenu /nomutex
Now, launch as many Gtalk instances as you want. Voila! Have fun