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 e-mail. 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. Another well-known example is the Friendship Screen Saver, also known as Yaha.E. However the well-publicised Budweiser Frogs screen saver virus turned out to be a hoax.

Logic Bombs and Time Bombs

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. This means that files will be deleted if she is ever fired. A company named Omega Engineering lost millions of dollars as a result of a logic bomb left by a former employee.
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 13th, deletes all the files on an infected computer.

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 way from a virus.
A worm will usually release a document that already has the worm macro inside it. The entire document travels from system to system, so the entire document should be regarded as a worm. W32.Mydoom.AX@mm is an example of a worm. It was designed to allow spammers to send spam e-mail from infected computers
Like Trojans, worms are self contained programs that are designed to copy themselves from computer to computer. Different worms exploit different weaknesses in the programs that allow computers to communicate with each other.
A common purpose of worms is to install a backdoor into a computer. These are programs that allow others to gain access to your computer.
You can find out more about the differences between viruses, Trojans and worms by visiting the following web site: Viruses, Trojans and Worms.

Trojans

A Trojan is a virus that hides inside another program. Named after the Trojanhorse of Greek mythology, a computer Trojan disguises itself as something else in order to gain access to your computer.

Trojans normally claim to do something useful but are actually malicious. One major difference between Trojans and true viruses is that they do not replicate. Trojans contain malicious code that can cause loss or theft of data. Trojans must be invited onto your computer, eg: by opening an email attachment or downloading and running a file. Trojan.Vundo is an example.
There are two types of Trojan:
A program that pretends to do something useful but is actually a virus
A real program that has been altered to contain a virus
Trojans differ from other types of virus in that rather than attaching themselves to existing programs they are complete programs in themselves.

Types of Virus

There are several different types of viruses:
File infector viruses: these infect program files, such as applications, games or utilities. They are often memory-resident, meaning that once they have been executed they remain active in the computer's memory and can infect more programs. Examples include Jerusalem and Cascade.
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. Boot sector viruses attach themselves to this program and execute when the computer tries to start up from an infected disk. Once a computer has been infected, any unprotected floppy disk put into the computer will also be infected. Infected machines will often refuse to start. Examples include Michelangelo and Stoned.
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. Examples include Anthrax and Tequila.
Macro viruses: these infect data files, such as Word documents or Excel spreadsheets, rather than programs. They are very common and can be difficult and expensive to repair. Macro viruses are written using macro programming languages, designed to allow users to automate tasks within an application. They are easy to produce, so there are now thousands in circulation. Examples of macro viruses include W97M.Melissa and WM.NiceDay.

Viruses, Trojans and Worms

The words Trojan, worm and virus are often used interchangeably, but they are not the same. They are all malicious programs (sometimes called "malware") that can harm your computer, but there are important differences among them.
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. Some viruses are a threat to system integrity: they damage computers by altering programs, deleting files or other malicious activities. Others simply announce their presence by audio, video or text messages.
Even viruses which don't do any real damage are a nuisance as they can reduce system performance by wasting memory or slowing down a computer. Most viruses can cause a system to behave oddly and some can cause the system to crash, often making it difficult to restart. Some viruses are a threat to data security, as they attempt to steal confidential data from a machine and transmit it to someone.
Viruses can spread by many methods, including attaching themselves to other files such as programs and documents. Viruses are given different names depending on the way they spread. Some of these different types of virus are explained in later sections.
Simply spreading from computer to computer is not itself a major problem. Most viruses contain a payload, a program that they will execute in addition to spreading themselves. The effects of common virus payloads include deleting or corrupting files or disks and passing on usernames and passwords.

introduction to viruses

Most people are aware of threats to their computers. Viruses, Worms and Trojans have become so common that new stories about computer threats appear daily in newspapers and on television. However, you may not realise is that these threats are relatively new. Two things have happened in recent years to make computer threats possible:
-Computers became affordable and therefore common
-The Internet allowed computers to connect easily to each other

This software was the first to copy itself from computer to computer, infecting all the machines it came in contact with.

Some people think that only PCs (IBM clones) are at threat and that Apple Macintosh (Mac) and Unix/Linux systems are immune to threats such as viruses. This is not entirely true. Other computers systems are often more secure than PCs. However, the main reason that more PCs are affected by viruses is simply that there are more PCs in the world.