If you are like a lot of people and don't have a checkbook program or business information on the computer a good reformating would probably be more than secure enough.  BUT if you have any financial information, proprietary information nobody else needs to know, passwords (always assume the computer stored everything someplace) the only good answer is remove the hard drive and either store it or destroy it.

When XP came out I bought several computers at yard sales very cheap.  Usually what happened is I bought a monitor or peripherals for a fair price and the computer came along.  It was  interesting seeing what kind data people let slide through their fingers to a stranger.