Here is how to start safe mode at boot. Please post exactly what you changed, it will make this much easier. What game? Where did you get the advice from? I know of no game(s) that require this. You may have been scammed. Windows will not run on one core as you found out.
Reboot the computer. Start tapping (tap pretty fast..don't stop) the F8 key as soon as your manufacturer’s logo appears. This will open the Windows Advanced Options Menu.
If you begin tapping the F8 key too soon, some computers display a "keyboard error" message. To resolve this, restart the computer and try again.
If you see the Windows logo, it means you did not start pressing the F8 button soon enough. You will need to reboot and try again.
If you have more than 1 HDD you may get a screen asking which to boot from. Select your boot drive but you have to keep tapping the F8, it can be a bit tricky.