GTalk Trick (it works, I tried it out):
If you have two or more Google accounts, and want to use both at same time for chatting with friends, then this is the perfect trick for you. With this trick, you can run multiple copies of Google Talk application in you Windows.
All you need to do is to right click on Google talk shortcut and select Properties. In the target field append the parameter /nomutex so that it becomes like C:/Program Files/Google/Google Talk.exe /nomutex. It’s done. Now whenever you double click on Google Talk Desktop icon, it opens a new instance for a new Google account.