You can use KillDisk or DBAN to completely wipe the hard drive. Note, though, that without an OS CD you have no way of reinstalling an operating system, meaning the computer would be unusable until you either find your Windows CD or go for a Linux distro.
I have reason to believe a problem computer is about to fry itself but I have no OS disc to help me reformat. Before all I had to do was throw in a '98 or XP disc and it did everything by itself. Is there any way I can flush everything on that computer down the toilet, to be formatted later when I decide between XP home or pro?