if you've got partition magic, then use that. otherwise fdisk works just fine.
fdisk, select hard disk if you've got more than one hd (5), delete partition (3), non dos partition (4), ...
if you've installed lilo, you'll want to fdisk /mbr to get lilo out of mbr... (if you're not sure, just do it anyways... shouldn't do any damage to other partitions/os's).