A thief can snatch an unattended laptop from a desk and walk away in a matter of seconds. And a thief who has your laptop may have access to your files and personal information. A notebook lock won't prevent someone from cutting the cable, but it can deter crimes of opportunity.
Kensington is probably best-known for its notebook locks; it offers an array of locks for laptops and desktops. Targus is a second vendor that specializes in laptop security gear, including one lock that sounds an alarm when someone tries to pick up the attached laptop or cut the lock cable.
Prying eyes are a common security hazard. To prevent unauthorized viewing of your data when you step away from your desk, always lock your screen before leaving your PC unattended. To do this, simply hold down the Windows key and type the letter L. This will bring up the lock screen. To get back to work, press Ctrl-Alt-Delete, and enter your login password at the prompt.
Another way to shield your screen is to install a privacy filter over the display. These filters fit directly on a monitor so other people can't peer over your shoulder and see what's on the screen. A privacy filter may be particularly useful if you work in an "open" office that lacks cubicle walls. Various companies sell these filters, including Targus, 3M, and Fellowes.