Reinstalling Windows XP new.?

Alright, my windows XP is infected with a virus that cant be gotten rid of by antivirus software. aka. ive tried everything i could. The only way to get rid of it is to reinstall windows new. I only have one partition i can install it on. It said if you install it in a partition that already has another operating system, it could make the computer not run properly. I basically want to overwrite it and delete everything and install windows new. How do i do this? As I've said, I've went into the setup from boot and it said i have a damaged operating system. Do you want to repair it? Is that what I'm supposed to do? I have to make sure so I don't screw up my computer, I cant get another one for a long time. It's the same operating system I'm installing, Windows XP, but it's just a newer version. What is it need to do to delete that old operating system and install this one? Like I said I want to install it from COMPLETE scratch.