Ok, so first of all you must know that if you do something wrong, you might damage your system.So first of all you
must restart your PC.After you that you must enter the bios and under cpu advanced settings(notice that i don't know your bios type, because the bios might vary from the motherboard producer) you must look for the cpu frequency in your case, you must look for this number:266 Mhz, this might be the stock clock speed.To increase or decrease it press the num + or num -. First you must increase it a bit and then, when the OS boots try running a stressing application to test the cpu. Repeat till you encounter a BSOD. If you encounter a BSOD while running a stressing application try decrease the clock frequency till it's all stable.(Notice if you do that you'll have a different clock speed and a maximm overclock limit. If you want to overclock it more try changing your cpu cooling method, buying a better socket 775 cooler or water colling but for more money). I hope I helped
