A Virus is a piece of software code or program that is written specifically to infect a computer. The effects of a virus can range from humorous text being displayed on your monitor to the destruction of all your files. 

In the old days before the Internet a virus was only able to spread itself via floppy disk. The preferred method now though is e mail. The virus is sent as an attachment to an email and it's payload is delivered when the unsuspecting victim opens the attachment. Often the sender will put a message designed to intrigue you so that you will open it. The most famous virus was the I Love You virus which caused world wide chaos. As soon as this attachment was opened it scanned your contacts and e mailed itself to everybody in there. The e mail then looked as though you had sent it. 

The best defence against these kind of virus is common sense. Even if you had the latest virus checker installed it would not have stopped the I Love You virus. It was hours before a cure had been found and posted on the internet. If you are not expecting an e mail from somebody or it looks suspicious DON'T OPEN IT. As yet there are no virus in the wild that can execute themselves without being opened first. 

That said it is vital that you have a virus checker installed on your system and that you regularly update the definitions file. If you don't have one then you can down load one by following the link below. You have to register but it will not cost you a penny and the definitions file is updated almost daily so you will always be up to date.