the winxp disk when loaded give you the option to revert back to a previous operating system..anyway options are download a win98 boot disk from
www.bootdisk.com change the boot sequence in the bios <del or ctrl keys>boot from cdrom or completly wipe the hard drive by using this>
http://www.killdisk.com/ when you upgraded to winxp it would have been a doodle re-loading win98 if fat32 was used..never mind try the killdisk progam..