Firstly, you don't format the cpu, you format the hard drive.
Why do you feel the need to reformat? formating the drive deletes everything on it, leaving you with an inoperable computer.
The operating system would need to be reinstalled, do you have a instalation disk to reinstall it with? Drivers would also have to be reinstalled for your hardware.
Giving you instructions on how to format at this time could do you more harm then good.
Maybe post back with more information about why you need to. I wouldn't like to see you stuck with a useless pc.