If you're selling or giving away an older computer or laptop, you want to make sure that you securely wipe all the data off its hard drive. Simply formatting the hard drive is not enough these days. To really protect yourself you need to use a special program that can securely erase all the data off the drive.
When you format any type of media, the OS will only erase the media's Master File Table (MFT). The MFT tells the OS where all the parts of a file are stored across the disk. Since the data is not erased, it's possible to recover it from the hard drive.
To really protect your personal and financial information you need to use a disk wiping program that can securely erase every bit of data off a drive by writing random information over it several times. There are a few free programs that can perform this type of operation for you.
Secure Disk Eraser: Darik's Boot and Nuke (Erases the whole hard drive)
Secure File Eraser: WipeFree (Erases individual files)