Just follow few steps as shown below...
1. Place a google talk icon on your desktop & Right-Click Select properties from context-menu navigate to "SHORTCUT TAB" in this concentrate on the "TARGET" option by default it is having the address like this
"C:\Program Files\Google\Google Talk\googletalk.exe" /startmenu
2. All you need to do is just replace that address to below address
"C:\Program Files\Google\Google Talk\googletalk.exe" /nomutex
3. Click Apply and OK then your done.
Screen shot:
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