Hermetic Systems' Data Destroyer is a disk-wiping utility that can render data unrecoverable by overwriting it multiple times. It will scrub your hard disks, floppies, and ZIP disks clean. It will delete individual files, whole directories, or entire disks full of data. You can select the type and number of purges, or overwrites. Data Destroyer will even estimate how long it will take to perform the selected number of purges. It meets stringent U.S. Department of Defense standards for data disk sanitization, too. If it's good enough for the Pentagon, it'll probably meet your needs, too.