Viruses, worms and trojan horses can cause great damage. By knowing what you’re dealing with it is easier to protect your computer.

What is a virus?
Computer viruses are software programs that are deliberately designed to interfere with computer operation, record, corrupt or delete data, or spread themselves to other computers and throughout the Internet. Viruses vary from being troublesome to very destructive.

A virus does not spread without user intervention. Someone has to share a file or send an e-mail to spread it to other computers. Viruses can also spread through instant messaging, such as Windows Live Messenger.

What is a worm?
In the same way as viruses, worms are made to copy themselves from one computer to another. But worms do this automatically. This means that when they have entered a system, they can self-replicate in large numbers and copy themselves to different locations. They might send copies of themselves to everybody in your contact list, and then do the same thing on their computers. In networks with a lot of traffic this may cause a massive spread of a dangerous worm within a very short time span.

What is a Trojan horse?
The mythological Trojan horse appeared to be a gift, but in reality it contained several Greek soldiers that conquered the city of Troy. The modern Trojan horses however, are computer programs that appear to be useful programs, but instead they reduce your computers safety and cause a lot of damage. They also make it possible for criminals to control your computer via the Internet, change files and functions, monitor and register your activities, expose you to fraud and use your computer to attack other computers.

What is a zombie and a botnet?
Criminals can use viruses to control several computers at once. They can turn them into zombies or bots (robots) that work together in a powerful botnet that may consist of several thousand computers. These can distribute spam, spread viruses and attack other computers and servers. They can also perform other types of crime and fraud. Botnet is a very common phenomenon in electronic crime, and has become a serious problem on the Internet. Be aware of the fact that you rarely notice anything if your computer is included in a botnet.

How do I reduce the risk?

• Never download software form senders you do not completely trust.

• Never download software with “fun” or “valuable” functions that are offered for free or at a very low price.

• Look out for popup windows that claim to be antivirus software.

• Update your antivirus software, antispyware software and firewall regularly.

• Only use licensed software. Unlicensed software can be more exposed to virus, and these programs may be installed with a virus without your knowledge.

How do I know if a have a virus?
When you open or drive an infected program, you may not be aware that your computer has a virus. But there are some signs that give you a clear indication that something is wrong:

• Your computer slows down and does not perform as it used to.

• Your computer often crashes and restarts itself.

• Mysterious messages pop up or your computer responds in unexpected ways.

• When you turn on the computer the screen is black.

What do I do if my computer is infected?

• See to that your software and operating system is updated. If your operating system is Windows, then run the Microsoft Malicious Software tool (if you don’t have it, it is available here).  This software checks your computer for infections by specific, prevalent malicious software, and helps remove any infection found.

• Keep your firewall turned on. A firewall helps to protect your computer from hackers who might try to delete information, crash your computer, or steal your passwords or credit card numbers. Useful information about the different types of available firewalls and how you choose the right one is found here.

• Keep your antivirus software up to date. Antivirus software is a computer program that detects, prevents, and takes action to disarm or remove malicious software programs, such as viruses and worms.

• Keep your antispyware technology up to date. Antispyware helps protect your computer against pop-ups, slow performance, and security threats caused by spyware and other unwanted software.

Source:
Microsoft

Relevant links:
Microsoft Security - the latest in computer safety
Microsoft Online Security - online safety and privacy education