It's an unfortunate fact of computer science: when a computer program runs, it will do what it's programmed to do, even if it's programmed to be harmful . When you choose to run a program, you are making a decision to turn over control of your computer to it. Once a program is running, it can do anything, up to the limits of what you yourself can do on the computer. It could monitor your keystrokes and send them to a Web site. It could open every document on the computer, and change the word will to won't in all of them. It could send rude e- mails to all your friends . It could install a virus. It could create a "back door" that lets someone remotely control your computer. It could dial up an ISP in Katmandu. Or it could just reformat your hard drive.
That's why it's important to never run, or even download, a program from an untrusted sourceand by "source," I mean the person who wrote it, not the person who gave it to you. There's a nice analogy between running a program and eating a sandwich. If a stranger walked up to you and handed you a sandwich, would you eat it? Probably not. How about if your best friend gave you a sandwich? Maybe you would, maybe you wouldn'tit depends on whether she made it or found it lying in the street. Apply the same critical thought to a program that you would to a sandwich, and you'll usually be safe.