formatting the computer.
depending on what o/s you have:
xp and 2k -- instert your cd, follow instructions, you will eventually get to a spot where it will format
95/98 -- instert your boot floppy, when it is loaded type c:format, that will erase everything on your drive. then type a:fdisk and follow the instructions to delete your partition, then create a new one.
then, install your o/s
duel OS's
start with a clean HD
when installing your first OS (assuming xp or 2k) create two partitoins with the size your desire.
install the 1 os to the first partition
install the 2nd os to the second partiton.