Microsoft’s Law No.1 of 10 Immutable Laws of Security, according to its website:
"If a bad guy can persuade you to run his program on your computer, it's not solely your computer anymore. 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 a certain level of control of your computer to it -- often anything up to the limits of what you yourself can do on the computer (and sometimes beyond). It could monitor your keystrokes and send them to criminals eager for the information. It could open every document on the computer, and change the word "will" to "won't" in all of them. It could send rude emails to all your friends. It could install a virus. It could create a "back door" that lets someone remotely control your computer. It could relay a bad guy’s attack on someone else’s computers. Or it could just reformat your hard drive.
"That's why it's important never to run a program from an untrusted source, and to limit the ability of others to make that decision for you on your computer. 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't—it 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.
"Privacy and Security reminder: Do not open documents or click on links from untrusted sources. If the source is trusted, ask the trusted source where he or she obtained the document or link."