It might seem obvious, but we think it is worth pointing out that you should keep your computer as physically secure as you want its data to be.
If someone has physical access to your machine, she could boot it into single-user mode and then have the run of the system. (See Chapter 11, "Introduction to System Administration," for more on booting into single- user mode.)
And, of course, if someone has physical access to the machine, he might be able to steal it, install malicious software on it, or even install a hardware keystroke-recording device (see Privacy.org; www.privacy.org/archives/000990.html).
The main idea here is to remember that computers, hard drives , and wires all exist in the physical world, and that the same security precautions you apply to, say, your checkbook should be applied to your computer and your data.