OCing i5 2500k

First off just like any overclocking you will need to enter the bios. Depending on your motherboard I always recommend going to the Save/Exit section and select restore to Defaults just in case some settings were changed if you were adjusting things so we can start fresh.One thing that I always recommend is disabling all the things you are not using such as eSata, USB 3.0 etc. After that depending on your motherboard you should have an Overclocking section so we need to go here next.

First go ahead and set your CPU Multiplier between 40 and 45 Since the Baseclock on most P67 motherboards should be 100mhz, 100mhz x 40=4ghz etc. This is the option that we will use because Sandy Bridge is very testy when it comes to adjusting the Baseclock.

Next go to VDroop and change this to without Vdroop. This will basically help reduce sagging while we run Prime95 or Linx and keep the voltage stable.

Next go to Internal PLL Voltage Override and set this to Enable- This is a very important step whenever you are using a multiplier of 40 and greater.

Next go to CPU VCore and set this to manual and then I would recommend starting at 1.300V for 4ghz and if you want 4.5 you will more than likely need about 1.325 but always remember to test in small increments in order to achieve a nice stable overclock. This may vary on your CPU whether or not you have the same identical one as someone else.Now we need to configure some other very important features in our Bios which some are completely optional but I would highly recommend changing them for stability.

First go to EIST (Intel Speedstep Technology)and disable this feature. Basically this allows your CPU to throttle down below even the stock 3.4ghz when tasks are at a minimum so it is optional but I always disable it.

Next got to C1E Support and disable this feature. This is another power saving feature but enabling this might cause instability.

Next go to CPU C3 Support and set this to disable. I believe this is also called Sleep where the processor does not need to keep its cache coherent but maintains another state.

Next make sure you have Turbo Mode enabled. Most P67 -Z68 motherboards have this feature.
Next up, memory settings! Make sure to configure this per your memory specifications, or use the XMP function to use the built in SPD settings (if applicable)Now you can go ahead and Save/Exit to Windows. I would definitely recommend having some Temperature software previously installed on your system. I swear by Coretemp but there are many others out there. Make sure you keep a close eye on your temps after booting into windows.

Now that you are in windows check you clock settings with CPUZ and I recommend running a stability test such as Prime95. Another great one is LinX which is very intense. You can run it as long as you want and there are many different opinions as to how long you should run the tests to do some research online and make your decision.

That is basically it for now so if you have any questions feel free to ask me and I will be able to help you to the best of my ability.