How to erase everything off your computer?

How do you like erase everything on your computer? Like when it was brand new and had nothing downloaded?

Can it be simple or do you have to buy software?

You should have a CD titled 'System restore CD' or something very similar. Put in into your cd drive, reset your pc and then you should have the option to restore your PC to factory settings. But make sure you backup any data before you do this as anything you've installed, downloaded or stored on your PC will be wiped clean.

Reformat

You need the (assuming you are windows based) windows disk that came with your computer

If you don't have it, then you'll have to buy it and it gets pretty expensive

You could always just manually remove all programs and files that weren't originally part of the format, as well

You will need your Operating Systems cd. Like if you have XP, you will need YOUR XP cd. Hopefully you have a legal version and the cd. Otherwise I dunno if you can do it or not.