A virus is just a computer program. Like any other program, it contains instructions that tell your computer what to do. A virus instructs your computer to damage certain files and spread itself to other files on your computer and to other people's computers.
Viruses have been developed into various types and are given names such as worms and trojans. These are all malicious pieces of code designed to disrupt your computer, or to help hackers take control of information held on your hard disk.
Anti-virus software scans files on your computer to check for known viruses. If it finds any viruses it will try to remove them, normally advising you on the steps it has taken and offering advice on what to do next.
BT Business customers can guard against these threats by ordering BT PC Security.