Simple, you need a bootdisk so make one.

Boot into dos making sure your CD/ROM drive is loaded,
run FDISK and delete the necessary partitions . if your partitions were NTFS, then your in problems.
Coz FDISK normally only recognizes FAT & FAT16
it'll also only partition 2gb of space a time,
Once you've followed the instructions and restarted the machine you'll need to format the active partition and any other partitions u created using fdisk
from their own run the setup program from the OS cd which you want to install and bingo!