How do I completely remove Windows 98 from a computer?

My laptop died a little while ago (someone of the more diehard Flutheranians may remember that from a previous question) Soon, I’ll be purchasing a new one, but in the interim, I’m using my Dad’s old one (Compaq Armada 7400, Pentium II) It’s saddled with Windows 98, and I’m wanting to make it a bit more useful and install Windows XP. Whenever I try this, however, it tells me that 98 can’t be upgraded to XP. So, I want to start from scratch and dump 98 altogether, then install XP anew. But I can’t figure out how to remove 98, as the XP installation won’t give me the option to remove the current operating system.