Lets begin by clicking on 'start' in the bottom left and clicking "Run". In that pop-up, type "msconfig", click "ok" and you should get a pop-up similar to the following image. Then click on the "BOOT.INI" tab at the top. Next you will need to press the "Advanced Options..." button. and should see the pop-up similar to the image to the left. Now you should check the check box before "/NUMPROC=" and set the drop-down box to the highest number. This means that your computer will now boot using the number of cores specified.
Windows Vista / 7:
Lets begin by clicking on 'start' in the bottom left and typing "msconfig", click enter and you should get a pop-up similar to the following image. Then click on the "Boot" tab at the top.
Next click on "Advanced options..." and you should see something similar to the following image:
Now, check the box that says "Number of processors:" and then click the drop-down box beneath it and specify how many cores you want to boot from.
Note: If you do not see the options in the previous images, then you are only running a single core processor. Also note that every computer is different. This may help some of you and other may see no effect.