You’ve got multiple GMail accounts, say, one for work and another for personal use; and you would like to sign onto both Google Talk accounts on the same computer at the same time. By default, GTalk doesn’t permit multiple instances, but a simple hack can fix it.
Using A Simple Hack
Right Click on the Google Talk shortcut icon in the desktop, in your quick launch or by going to Start > All Programs > Google Talk folder > Google Talk , then click Properties.
add /nomutex at the end of the line, for example:
C:\Program Files\Google\Google Talk\googletalk.exe” /startmenu /nomutex
Make sure that there is a space between /startmenu and /nomutex
Now you can open multiple Gtalk messenger windows by clicking the program multiple times, and use each messenger for different account.
Perform Hack Using Command Prompt
Click Start and hit Run, type CMD in the run window, now go to the directory where Google talk is installed and add /nomutex as in C:\Program Files\Google\Google Talk>googletalk/nomutex at the end as shown in screenshot below.
Download Software To Use Multiple Clients
If all else fails, then use this step. Using this step will give you a little extra feature but it will require a multi-protocol IM client to be downloaded called Digsby.
It will allow you to add more than one, in fact even up to 7 or more Google Talk accounts easily. It has some amazing features like the ability to add different social networks, different email accounts.
That’s it. All three methods stated above works just fine for me, if you are having any trouble, please let me know in the comments.