How to get rid of an OS?

[Hello]

I have just used a recovery cd, and rather than deleting my old one like I wanted, I somehow have 2 OSes on here. From my new one I have all kinds of stuff on here from my old OS that I do not want, how do I reformat with completely starting over, as if this computer were just out of the box? Thanks in advance

 

[__Camouflage__]

Download MaxBlast4 and burn it to a CD as an ISO, boot from it the CD, and format with it.