What do viruses do to computers?

Viruses are software programs, and they can do the same things as any other  programs running on a computer. The actual effect of any particular virus  depends on how it was programmed by the person who wrote the virus.  Some viruses are deliberately designed to damage files or otherwise  interfere with your computer's operation, while others don't do anything but  try to spread themselves around. But even the ones that just spread  themselves are harmful, since they damage files and may cause other problems  in the process of spreading.  Note that viruses can't do any damage to hardware: they won't melt down your  CPU, burn out your hard drive, cause your monitor to explode, etc. Warnings  about viruses that will physically destroy your computer are usually hoaxes,  not legitimate virus warnings.

 Articles From COMPUTER VIRUS FAQ

What kind of files can spread viruses?

Viruses have the potential to infect any type of executable code, not just 
the files that are commonly called 'program files'. For example, some 
viruses infect executable code in the boot sector of floppy disks or in 
system areas of hard drives. Another type of virus, known as a 'macro' 
virus, can infect word processing and spreadsheet documents that use 
macros. And it's possible for HTML documents containing JavaScript or other 
types of executable code to spread viruses or other malicious code. 

Since virus code must be executed to have any effect, files that the 
computer treats as pure data are safe. This includes graphics and sound 
files such as .gif, .jpg, .mp3, .wav, etc., as well as plain text in .txt 
files. For example, just viewing picture files won't infect your computer 
with a virus. The virus code has to be in a form, such as an .exe program 
file or a Word .doc file, that the computer will actually try to execute.

Articles from COMPUTER VIRUS FAQ

How do viruses spread?

When you execute program code that's infected by a virus, the virus code 
will also run and try to infect other programs, either on the same computer 
or on other computers connected to it over a network . And the newly 
infected programs will try to infect yet more programs. 

When you share a copy of an infected file with other computer users, 
running the file may also infect their computers; and files from those 
computers may spread the infection to yet more computers. 

If your computer is infected with a boot sector virus, the virus tries to 
write copies of itself to the system areas of floppy disks and hard disks. 
Then the infected floppy disks may infect other computers that boot from 
them, and the virus copy on the hard disk will try to infect still more 
floppies. 

Some viruses, known as 'multipartite' viruses, can spread both by infecting
files and by infecting the boot areas of floppy disks.

Article from COMPUTER VIRUS FAQ

How To Avoid Computer Virus

1. Install anti-virus software from a well-known, reputable company, 
UPDATE it regularly, and USE it regularly. 

New viruses come out every single day; an a-v program that hasn't been 
updated for several months will not provide much protection against current 
viruses. 

2. In addition to scanning for viruses on a regular basis, install an 'on 
access' scanner (included in most good a-v software packages) and configure 
it to start automatically each time you boot your system. This will protect 
your system by checking for viruses each time your computer accesses an 
executable file. 

3. Virus scan any new programs or other files that may contain executable 
code before you run or open them, no matter where they come from. There 
have been cases of commercially distributed floppy disks and CD-ROMs 
spreading virus infections. 

4. Anti-virus programs aren't very good at detecting Trojan horse 
programs, so be extremely careful about opening binary files and Word/Excel 
documents from unknown or 'dubious' sources. This includes posts in binary 
newsgroups, downloads from web/ftp sites that aren't well-known or don't 
have a good reputation, and executable files unexpectedly received as 
attachments to E-mail or during an on-line chat session. 

5. If your E-mail or news software has the ability to automatically execute 
JavaScript, Word macros, or other executable code contained in or attached 
to a message, I strongly recommend that you disable this feature. 

6. Be _extremely_ careful about accepting programs or other files during 
on-line chat sessions: this seems to be one of the more common means that 
people wind up with virus or Trojan horse problems. And if any other family 
members (especially younger ones) use the computer, make sure they know not 
to accept any files while using chat. 

7. Do regular backups. Some viruses and Trojan horse programs will erase or 
corrupt files on your hard drive, and a recent backup may be the only way to
recover your data.

Article From COMPUTER VIRUS FAQ

Types Of Viruses

Boot sector viruses are usually transmitted when an infected floppy disk is left in the drive and the system is rebooted. The virus is read from the infected boot sector of the floppy disk and transferred to the master boot record of the system's hard drive. Then, whenever the computer is booted up, the virus will be loaded into the system's memory. Any floppy disk formatted as a bootable disk in the infected machine would contain the virus and spread itself to other machines.

Program or file viruses are software programs that attach themselves to executable programs. Once the infected program is run, the virus is transferred to your system's memory and may replicate itself further. It may spread to all the files on a hard drive or to files of a specific type.

Macro viruses are currently the most commonly found viruses. They infect files run by applications that use macro languages, like Microsoft Word or Excel. The virus looks like a macro in the file, and when the file is opened, the virus can execute commands understood by the application's macro language. The infamous Melissa virus is a macro virus as well as the common Laroux Excel spreadsheet virus.

Multipartite viruses have characteristics of both boot sector viruses and file viruses. They may start out in the boot sector and spread to applications, or vice versa. 

Article from PCHELL

Computer Virus

Computer viruses are software programs that invade a computer system to wreck havoc with computer files or the computer users themselves. Some are benign causing nothing more than a nuisance while others can delete files or even destroy computer hardware. Typically viruses are programmed to reproduce themselves and spread from one computer system to another, just like a normal human virus spreads via human contact throughout the population. Computer viruses can be spread via diskettes or other removable media, by way of email attachments, or in some cases simply by being connected to the Internet and not protected.

Article from PCHELL