Intruders don't always use technology to crack into a network. Sometimes they use brute-force methods. For example, if a thief steals your laptop computer, he can often use the "convenience" features of the computer's software to connect to the Internet through your network, read your email and other confidential files, and gain access to other computers connected to the same network. Other low-tech methods include shoulder surfing, where the bad guy watches your screen or your keyboard as you type a login name and password, and social engineering, in which the intruder convinces you to reveal your password or other information because she has convinced you that she has a legitimate use for it.
There are some things you can do to protect your computer and the data stored in it:
Think like a thief and do whatever might be necessary to make the computer difficult to steal. Don't leave your laptop in plain sight in an unattended car, or walk away from it in a public location such as a coffee shop, airport, or library. In an office with cubicles or "open plan" workspaces, use a chain and lock to secure the computer to a desk, file cabinet, or other large object if you can't store it in a locked drawer or cabinet.
Don't place your computer near a window where passers-by can see exactly what you have.
When you're away from home or work, keep track of your laptop computer at all times.
Log off your office computer whenever you are about to leave your desk. Don't allow other people to wander in and use your account when you're not there.
Keep the login and password active on your laptop, and use encryption for files that contain confidential data. If somebody does steal the computer, you want it to be as difficult as possible for the thief to open and read your files.
Install an antitheft alarm device on a PC card in your laptop or an internal card in a desktop system. If somebody tries to move the computer, disconnect cables, or open the case without entering a security code first, the alarm device will sound a very loud alarm.
Use a tracking program such as LostPC (http://www.lostpc.com/) on your laptop computer. The tracking software will automatically send a "here I am" signal to a security center every time it connects to the Internet from a new location. Police and Internet service providers can sometimes use this information to locate and recover a stolen computer.
Consider using full-disk encryption tools such as TrueCrypt for Windows or dm-crypt for Linux, so the data on it is useless if your laptop is stolen.