Before selling a second-hand computer to someone else, it's always a good idea to wipe the hard drive completely.
Hard Drive Eraser can help you with this task. This standalone utility deletes the contents of any hard drive on your system in such a way that it's impossible to recover the data. It includes four different wiping methods, ranging from the most simple one, Write zeros, to the safest – and slowest – Peter Guttman method that overwrites data in 35 passes. The four different methods are explained in the program's Help section.
Hard Drive Eraser also works with any removable drive you connect to the PC, which makes it especially handy when it comes to erasing USB memory sticks before lending them to someone else. You should note also that partitions are listed as separate drives, so make sure you double check that you have selected the right one.
It may seem a silly piece of advice, but remember to make sure you want to delete those files before actually using Hard Drive Eraser on any drive. The program does ask you twice for confirmation, just in case.
Hard Drive Eraser wipes out all the content in your hard drive or any other removable drive, and makes sure it cannot be recovered.