Before disposing of, or donating, old computers, hard drives, CDs, computer tapes, or other electronic storage devices, make sure that all the data is destroyed.
A simple delete by dragging the file to the trash and then emptying it is not sufficient. Computers don't actually get rid of the data deleted this way, but simply mark the space as available for subsequent use. Meanwhile the old data is still "squatting" on your hard drive until your operating system happens to assign a new file to the space.
See Computer Recycling and Disposal Options at Penn.