Unable to uninstall Windows XP

Hello, Setup cannot uninstall Windows XP because the number of hard drives present after upgrading is not the same as the number present before upgrading. What does this mean? I had an external hard drive connected but have disconnected it and I cannot find it in the My Computer as only the "C" drive, floppy and CD drives are listed. I have the option of removing my previous operating system. What I really want to do is wipe the hard drive clean and go back to Windows 98.

Thanks for any help you can offer.