Computer security

What is a computer virus?

Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation.

A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.

Viruses are most easily spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it's from and you are expecting it.

Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files.

Viruses also spread through downloads on the Internet. They can be hidden in illicit software or other files or programs you might download.

To help avoid viruses, it's essential that you keep your computer current with the latest updates and anti virus tools, stay informed about recent threats , and that you follow a few basic rules when you surf the Internet, download files, and open attachments.

Once a virus is on your computer, its type or the method it used to get there is not as important as removing it and preventing further infection.

Signs of viruses: Are you infected?

After you open and run an infected program or attachment on your computer, you might not realize that you've introduced a virus until you notice something isn't quite right.

Here are a few primary indicators that your computer might be infected:

•	Your computer runs more slowly than normal
•	Your computer stops responding or locks up often
•	Your computer crashes and restarts every few minutes
•	Your computer restarts on its own and then fails to run normally
•	Applications on your computer don't work correctly
•	Disks or disk drives are inaccessible
•	You can't print correctly
•	You see unusual error messages
•	You see distorted menus and dialog boxes

Tip: Beware of messages warning you that you sent e-mail that contained a virus. This can indicate that the virus has listed your e-mail address as the sender of tainted e-mail. This does not necessarily mean you have a virus. Some viruses have the ability to forge e-mail addresses.

Virus

The term “virus” is often loosely used in reference to any type of malicious program, or it is used to describe any negative event that a malicious program causes to a host system.

In the simplest terms, a virus is defined as program code that replicates from one host file to another. This simple definition

leaves room for further sub-division, which has become necessary due to the evolution of malicious code over the last two decades.

Computer Viruses can be further classified by the types of objects they infect, the method used to select a potential host, and infection technique.

Infection by type: Boot sector and multipartite viruses infect boot sectors and key operating system startup files (primarily COMM

AND.COM).

File viruses infect application .COM and .EXE files. Word Macro and Excel Macro viruses infect Microsoft Word .DOC and .XLS files, respectively.

Classified by the method they use to select their host: “Indirect action file viruses” load into memory and hook into the system interrupt table(s) so they can infect as files are accessed. Conversely, “direct action file viruses” do not become a memory resident, they simply infect a file (or files) when an infected program is run.

Infection technique: “Appending viruses” add code to the end of a host file, while “Prepending viruses” insert thei

r code at the beginning of a host file, effectively "shifting up" the program's original code. Overwriting viruses replace the host file completely with their own code causing irreparable damage to the original host file. By contrast, companion viruses and link viruses avoid adding code to a host file at all.

Companion viruses create a file of the same name, but with an extension that is higher up in the execution hierarchy. Link viruses manipulate FAT (file allocation table) entries.

There are viruses that fail to w

ork altogether. This could due to a bug in the original programming of the virus or a natural corruption (for example, a devolving virus eventually corrupts itself to the point that it can no longer function). One wonders

how such corruptions can be classified as viruses at all, and yet they are the bane of the anti-virus industry. Corrupted samples show up all too often in well-intended comparative reviews, and can badly skew tes

t results.