FDISK
Delete Partition(s)
Delete Primary DOS Partition
Make sure you say yes and enter the Volume Name if required
Reboot
It'll say no OS installed (without the Disk in the drive)
You'll need a Full Install Version of Windows 95, 98, 98se, Linux, ME (EVIL), XP Home, XP Pro, or whatever other OS you might want to use (except Mac of course)