Viruses, Trojans and Worms The commonest computer threats are viruses. Virus A virus is a computer program which changes the way in which the computer operates without the knowledge of the user. A virus must execute itself and it must replicate itself, or make additional copies of itself. Viruses can spread using many methods including attaching themselves to other files such as programs and documents. The effects of common virus payloads include deleting or corrupting files or disks and passing on usernames and passwords.

Types of Virus File infector viruses: these infect program files, such as applications, games or utilities. Boot sector viruses: floppy disks and hard disks store a small program known as the boot record which is run when the computer starts up. Multi-partite or polypartite viruses: these infect both boot records and program files and are very difficult to repair as the virus code must be removed from both locations. Macro viruses: these infect data files, such as Word documents or Excel spreadsheets, rather than programs.

Trojans A Trojan is a virus that hides inside another program. Named after the Trojan horse of Greek mythology, a computer Trojan disguises itself as something else in order to gain access to your computer. Trojan horse of Greek mythologycomputer Trojan Trojans normally claim to be something useful but are actually malicious. One major difference between Trojans and true viruses is that they do not replicate.

Worms Worms can replicate themselves from computer to computer without infecting a host program, unlike viruses, which must be carried by a host. Although worms are often carried inside another file, usually a Word document or an Excel spreadsheet, they use the host in a different manner from a virus.

Logic Bombs and Other Threats Like other viruses, logic bombs are designed to cause harm to your computer. Logic bombs, however, only trigger when specific conditions are met. For example a computer programmer may insert code into a payroll program that deletes files if her name is not found on the list. In other words the files will be deleted if she is ever fired. Other viruses, called time bombs, may only detonate on a specific date or time. An example of this is the Jerusalem virus which, every Friday the 13 th, deletes all the files on an infected computer.Jerusalem virus

Screen Savers On a Microsoft Windows computer a.SCR file is a screen saver. As screen savers are just computer programs they are a useful way of inserting a virus into your computer. Typically SCR viruses come as an attachment to an . The text urges the recipient to click the attached file by promising perhaps a funny cartoon or enticing picture. Once clicked the virus then installs itself onto your computer. As the SCR file pretends to be something else it is another example of a Trojan. Perhaps the most famous example of a SCR virus is the SOBIG worm, which installed software to send spam.SOBIG

Resident Virus Some viruses only attack your system when the program that conceals them is run. Other, more sophisticated, viruses stay in your computer's memory ready to attack anything that is run on the computer. These are resident, or memory-resident, viruses. The concept of a resident virus is simple. Your computer runs many programs at the same time. This is called multi-tasking. At any moment you may have several programs such as a word processor, a web browser and a media player and many others running simultaneously in your system.resident virus