Originally posted by: mechBgon
To overwrite "empty" space on a drive with Win2000 or WinXP, open a command-prompt window and use the command cipher /w:C:\ (where C:\ indicates the C: drive, use the drive letter you want) and Win2000 or WinXP will write three layers of encrypted junk over the top of all available free space (and then delete it again).
I don't know if it qualifies as military-class or anything, but it does what you appear to want. How long it takes will depend on the speed and capacity of your drive. I just launched it on an 18GB 15000rpm SCSI drive and it looks like it'll take about five minutes to complete.