astrofreke 0 Newbie Poster

I have looked it up and it says that when you want a clean install of Windows XP, you go into the os disk and press repair or install. i went into the os disk and it says that i can install it but when i press it, it says that i cannot install because the disk is an older version than what's on the computer. Can someone explain what i need to do to reformat and reload my computer?