I just stumbled upon a site that told me about how to launch multiple GTalk windows and log in as different users with different GTalk windows.
Things you need to do:
1. Create a shortcut for GTalk on your desktop or Quick Launch bar.
2. Right click the icon and then in the Target Field, edit the value to add a new parameter like this.
"C:\Program Files\Google\Google Talk\googletalk.exe" /nomutex
Note: The only thing you need to add here is "/nomutex" after the default Target.
Now from this shortcut, you can launch different GTalk clients and log-in with separate user accounts. Happy GTalking.