How do you install windows 98 on a windows xp computer?

i have a windows XP computer with a 6.00GB hard drive, and with a 80GB slave drive. i found an old windows 98 installation file on the computer, but when i run it it shows "cannot run in this version of windows. start in MS-DOS mode." i have no windows 98 disk, just a windows XP disk. how do i install windows 98 on that computer on the 80GB hard drive? i don't have any disks, or anything. i have a disk labeled "Windows 98 starts here", but i think its just training. all i have for windows 98 is an old file on the windows XP hard drive (because that hard drive originally came from a windows 98 computer), that is the windows 98 setup file. how do i get it to install?