Theoretically, by default all the cores of a multi-core processor are activated when you start Windows 7, but in some cases they may not start. It is therefore necessary to force Windows to use all available cores. Here is a tutorial on how to force Windows to use all the cores when the system starts.
Click the “start” button then enter “msconfig ” in the search box and press the Enter key:
Click the Boot tab and then click the Advanced button:
An options window will open for advanced boot options, Check the Number of processors and select 2 from drop down box and then click OK button:
You have to restart to apply the changes done.