Viruses are computer programs which copy themselves and infect the computer without the user's knowledge. One of the many ways viruses are spread from one computer to another is over the network or Internet.

These viruses infect file systems on the network and when another computer accesses the file system, it too gets infected. These online viruses come in the form of 'worms' and 'Trojan horse', just to name a few. These are the two common viruses that are transmitted online. 'Worms' spread themselves to other computers without being part of a host, while a Trojan horse may seam harmless.

Yet both have vastly different damaging effects. Worms do not actually harm a computer system's hardware or software, while a Trojan horse can be extremely harmful in more ways than one when executed.

Viruses come in various forms unknown to many people. Most are file downloads uploaded by other users, or commonly known as file-sharing. The problem with these files is that they are not copyright protected, thus have no certificate. This makes it easier for viruses to infect the file. Users also have difficulty in deciding whether to download a specific file because a virus can infect almost anything, especially online software products that claim to be free.

To protect your computer from viruses, the most basic thing that can be done is installing anti-virus software. Different anti-virus software can detect different viruses. You may find that when you change anti-virus software on your computer the new anti-virus may detect virus on your computer that has been there for a long time but never picked up by your old anti-virus. So be sure to check with the suppliers to determine which anti-virus to use with your computer.

If you find that you may not be able to remove a virus using your anti-virus software because its effect has been dealt deep into your system, you will need to use system restore. Only available on Windows ME, XP, and Vista, you can reboot your system to an earlier date, thus rendering the system.

Worse come to worse, you will need to re-install your operating system. This process reformats the operating system (OS) and installs the OS from its original media. This method is guaranteed to remove any virus on your computer, but you will have to re-install any software you have manually installed before.