If you are giving away your computer, selling it on eBay or simply discarding it, and want to make sure that no personal data is left behind, then you will need to safely erase everything, including the operating system. Luckily there is some free software that will help you to do that, WipeDisk.
WipeDisk erases physical and logical disks (complete harddisks or single partition) securely and fast, this tool overwrites the information sector-wise, from the first sector of the disk to the last one, this is particularly important to correctly delete data and stop computer forensic tools.
The erasing program supports 14 different wipe methods: 2 US Navy standards, the standard of the US Department of Defence, US Airforce and the NATO.
If you only want to securely wipe files and folders and not the whole hard disk, you can use WipeFile or Eraser, they both are free utilities to securely delete single files beyond recovery.