I found this on Yahoo and thought it might come in useful if you are thinking of selling a pc.
To erase data and all digital traces of it, you’re going to need extra software to do it, unless you physically remove the hard drive and deploy a mallet.
Before you start, remember to back up your files by either burning them on to DVDs, using an external hard drive or cloud-based online storage options like Dropbox, Microsoft SkyDive or Google Drive.
You should also ensure you have your Windows discs that came with your PC or at least the code on the back of the disc packet which proves ownership. Whoever uses your PC after you’ve erased the hard drive will need to re-install the Windows operating system.
Choose which hard drives to erase
Know your PC and discover how your PC is set up. Head to Control Panel and see what hard drives you have active on your PC.
Your home drive is usually ‘C’ but this may be divided into partitions and, depending on your PC, you may have two drives - one a standard hard drive and one a fast, flash-based memory drive frequently called a SSD or Solid State Drive. Erase both to be sure you've got rid of all your data.
Pick software to wipe the drive
There are many software programs which allow you to erase data on your PC but the best free one is called DBAN or Darik’s Boot and Nuke.
Use your CD/DVD drive
DBAN is supplied as an ISO file, which needs to be burned to CD or DVD as your computer will be operating from your CD or DVD drive in order to erase the hard drive, which cannot be active when you’re clearing all the data on it.
You may already have software that allows you to burn DBAN to a disc as an ISO file - right click on the file and see if your computer offers you the option to burn as an ISO file. An ISO file differs from a simple file being copied to a disc. An ISO file allows a computer to boot the disc automatically and run the program - in this case, it’s DBAN.
In order to download an ISO burning program, visit Free Iso Burner. Alternatively, you can try the popular Nero Burning Rom 12 software which isn’t free but is very easy to use and currently offers a 15-day trial.
No, I dont work for the software company :-)