Nice 'n' Clean
Problem: You have an old computer with personal data. You want to ditch or recycle it. You heard that delete does not really delete files. They still exist hidden on-disk.You're afraid if someone can undelete files, access bank account details, steal your identity along with grannies secret recipes and your favourite aunt's photo album. What do you do?
Solution: DBAN - Darik's Boot And Nuke. This utility will permanently wipe any existing from your disk and overwrite so no data can be undeleted or recovered. DBAN works by overwriting the whole disk with zeros or randam garbage. DBAN also overwrites the partition table, making data recovery virtually impossible.
DBAN is fairly easy to use. Download, burn to compact disk. CD in drive and reboot. (You may need to change the boot order in the BIOS). DBAN gets to work, re-arranging those bits. On older, slower hardware, it can take some time to wipe clean all disks. Consider running overnight. DBAN runs unattended and is useful for wiping several disks.
DBAN is a Linux derived product.
Another solution, though one I have not, tried is KillDisk, which is a windows product. I haven't used this product.