Windows XP : How do I restore Win 98 after an XP upgrade?

Hi all,
I have a computer that originally came with Windows 98. I used a Windows XP upgrade ** to upgrade to XP. It always worked fine, no problems.Now, I want to wipe the hard drive clean. I figured I would use the original 98 Restore ** that came with the computer, then use the XP upgrade ** again after that. However, after I ran the 98 Restore **, the computer won't boot properly. The screen pops up as XP even though I haven't done the upgrade yet, and then then the bootup fails and gives an error. It's like it didn't really format the drive and there were still remnants of XP leftover.What do I do?