Here are answers to some common questions about viruses, worms, and Trojan horses.
A virus is a program that replicates itself. It spreads by making copies of itself on a computer or by inserting computer code into program or operating system files. Viruses don't always damage files or computers, but they usually affect a computer's performance and stability. For a virus to infect a computer or to spread, you usually have to do something, such as open an infected e‑mail attachment.
Yes, to varying degrees. Virus damage can range from slowing computer performance to a loss of information and programs. In the worst case, viruses delete or modify information and programs on your computer. Some e‑mail viruses send confidential information in messages when spreading. Even when a virus is not directly damaging information, the process of replication can slow your computer and Internet connection.
A worm is computer code that spreads without user interaction. Most worms begin as e‑mail attachments that infect a computer when they're opened. The worm scans the infected computer for files, such as address books, or temporary webpages, that contain e‑mail addresses. The worm uses the addresses to send infected e‑mail, and often mimics (or spoofs) the "From" addresses in subsequent e‑mail messages so that those infected messages appear to be from someone you know. Worms then spread automatically through e‑mail, networks, or operating system vulnerabilities, often overwhelming those systems before the cause is known. Worms aren't always destructive to computers, but they usually cause computer and network performance and stability problems.
A Trojan horse is a malicious software program that hides inside other programs. It enters a computer hidden inside a legitimate program, such as a screen saver. It then puts code into the operating system, which enables a hacker to access the infected computer. Trojan horses do not usually spread by themselves; they are spread by viruses, worms, or downloaded software.
A blended threat is a combination of some or all of the malicious programs discussed in the sections above. Blended threats often begin when someone interacts with a virus (such as opening an unsafe e‑mail attachment). The virus automatically propagates using worm behavior, and then installs a Trojan horse on the infected computer.
Install an antivirus program. To be most effective, an antivirus program should always be turned on, should scan incoming e‑mail, and should scan file access. For more information, see How can I help protect my computer from viruses? You can also download Microsoft Security Essentials, a free antivirus program, at the Microsoft Security Essentials website, or visit the Windows Security software providers webpage to find a third-party antivirus program.
There is no single indicator of a virus infection, but some of the more common effects include poor computer performance, pop-up ads displaying (even when you have turned on a pop-up blocker or disconnected from the Internet). For more information, see How can I tell if my computer has a virus?
To check for viruses and remove them from your computer, you need to run antivirus software. For more information, see How can I tell if my computer has a virus? and How do I remove a computer virus?