reformatting troubles

I have a computer with two hard drives. On the first was Windows XP. I want to give this computer to my daughter. I wanted to reinstall xp thinking it would allow me to reformat hard drive in the process. It didn’t. In fact, it installed it on the other hard drive (I wasn’t paying attention…assumed c would be default). I can’t uninstall xp on either drive with “add/remove”. I think I deleted the uninstall files. I just want to reformat the c drive and start again, but every time I tryit says “unable to complete”. I assume this is because it has the OS on it. How can i overcome this? I tried to reinstall Win 98 but I can’t do that either.