In case it hasn't been mentioned yet, hit "Windows Key + R". In the run box that pops up type "msconfig.exe" and hit enter. This should start msconfig.
Click on the "boot" tab then click "advanced options". In the box that opens up, you'll see a tick box for "Number of processors". Normally, this should be unticked and the list-box under it should be grayed out. If you find it in any other condition, try ticking the box (if unticked) and setting the number of cores to zero and then reboot. If it doesn't allow you to set the number of cores to zero then set it to 8 and reboot.
