The first question is what to do with your old computer? If it's more than five years old and unable to run the latest software, the answer is probably to recycle it.
Whatever you do, don't throw your computer away. It contains a lot of materials that are hazardous in a regular landfill, including heavy metals such as mercury, cadmium and lead [source: Tsydenova and Begtsson]. Also, many of the materials in computer parts can be extracted and reused to make new products, reducing the need to mine new materials [source: e-cycle St. Louis]. All you need to do is remove your hard drive and take the other pieces to a local electronics recycling center.
If you have a newer computer, it would be better to sell or donate it. Decide whether to wipe the drive yourself or hire a professional to do the job. You can take your PC to one of Microsoft's certified refurbishers, who'll wipe it using state-of-the-art security methods [source: Microsoft].
For a Mac, just ship it to Apple and you'll get credit at the Apple store if the company determines the computer has some monetary value [source: Apple]. However, if you're a reasonably tech-savvy person, read on to learn how to clean it yourself.