just make a boot disk or download one from
www.bootdisk.com for win 98
then boot with it in the floppy drive and type FORMAT C: then type D: then type setup.exe and you will be in windows setup...follow the instructions from there
also use you harddrive and cd rom letters for C and D those are the most common though for win 98 atleast
if the boot disk does not work go into the bios and set it to boot the floppy before the hard drive
to get to the bios you need to press a key when it it booting up...if it dosen't tell you common ones are f1 f2 f10 f11 f12 delete and esc