Dunno if I have got some of the guys out here confused...
I do not want to encrypt anything.
I just want to DELETE everything from all my hdds - including C (which I will do later on another system).
I dont know much about the software that you are using but dd command in linux is dumb enough to overwrite all the data including MBR and partition tables on the disk.All the methods posted in this thread, will in one way or another, erase the contents of the hard disks. Most of the tools won't touch the partition tables, MBR, FAT entries etc. So technically, your hard disk will still contain bits and pieces of information which could be retrieved by your smart software to trace previous installations.