Not on about changing the number of cores?
1. Type “msconfig” (without quotes) in Vista or Windows 7 start menu search box and hit enter.
2. Click Continue if you get a UAC prompt.By default, you won’t get a UAC prompt in Windows 7.
3. Navigate to “Boot” tab and click on “Advanced options” button.
4. Check the box named “Number of processors” and also select the number of processors from the drop down box.
5. Click “Ok” and restart to see the time difference.
6. You are done.