u need a win98 bootdisk from www.bootdisk.com.
put it on a floppydisk and boot yer pc with this floppy. You have too set yer BIOS to boot from the floppy.
once you are booting from the floppy you need to select "boot with CD"
then a few minutes later u get this:
type "format c:" THIS WILL ERASE ALL DATA ON THIS PARTITION
assuming that c: is your windows partition
then, type "setup" and windows will install
I would reccomend using windows2000, its more stable etc
u need the win2000 bootdisks from www.bootdisk.com
hope this helps