1. Go to the Gtalk Program Propeties: Start>>Programs>>Google Talk>> Right Click on the Google Talk

2. Change the Target: For emample;
"C:\Program Files\Google\Google Talk\googletalk.exe" /startmenu to
C:\Program Files\Google\Google Talk\googletalk.exe" /startmenu /nomutex

3. Click Ok. Then you can open multiple Gtalk.