Keep your computer physically secure.
Enabling firewalls and keeping your operating system patched are very effective methods of protecting your data from threats on the Internet, but do not prevent someone from sitting down in front of your keyboard and gaining access to sensitive information. If you're walking away from your machine, use a screen saver with a password lock or simply log out of your operating system. Be suspicious of anyone you don't know attempting to gain access to a computer in your area.
Never assume physical safety, even in a swipe-access location. There is almost always a way for someone to get to a restricted area. Keep very sensitive data on monitored central servers and not on machines that can be carried away.
Columbia University's Department of Public Safety provides several products and services to assist users in securing their computers. Among these are discounted locks for laptops and desktops, discounted computer recovery software, and free property engraving in collaboration with the New York Police Department. If you're interested in these, you can contact Public Safety's Crime Prevention Office at 212-854-8513.
Columbia's Data Sanitization / Disposal of Electronic Equipment Policy states that, before disposing of an old computer, all data must be wiped from the hard drive using DBAN. DBAN, an open-source application that removes all data from hard drives, is compatible with PCs and Macs.
Download DBAN here or contact your local IT support team.