SPYWARE

Virus can be reprogrammed to do many kinds of harm including the following.
1.Copy themselves to other programs or areas of a disk.
2.Replicate as rapidly and frequently as possible, filling up the infected system’s disk and memory rendering the systems useless.
3.Display information on the screen.
4.Modify, corrupt or destroy selected files.
5.Erase the contents of entire disks.
6.Lie dormant for a specified time or until a given condition is met, and then become active.
7.Open a back door to the infected system that allows someone else to access and even control of the system through a network or internet connection.
8.Some viruses can crash the system by causing some programs (typically Windows) to behave oddly.
How viruses spread from one system to another?
The most likely virus entry points are email, Internet and network connections, floppy disk drives, and modems or other serial or parallel port connections. In today's increasingly interconnected workplace (Internet, intranet, shared drives, removable drives, and email), virus outbreaks now can spread faster and wider than ever before.

The following are some common ways for a virus to enter the users’ computer system:
•Email attachments
•Malicious scripts in web pages or HTML email
•FTP traffic from the Internet (file downloads)
•Shared network files & network traffic in general
•Demonstration software
•Pirated software
•Shrink-wrapped, production programs (rare)
•Computer labs
•Electronic bulletin boards (BBS)
•Diskette swapping (using other people’s diskettes for carrying data and programs back and forth)

High risk files

The most dangerous files types are:
.EXE, .COM, .XLS, .DOC, .MDB
Because they don't need any special conversion to infect a computer -- all they've got to do is run and consequently the virus spreads. It has been estimated that 99% of all viruses are written for these file formats.
A list of possible virus carriers includes:
EXE - (Executable file)
SYS - (Executable file)
COM - (Executable file)
DOC - (Microsoft Word)
XLS - (Microsoft Excel)
MDB - (Microsoft Access)
ZIP - (Compressed file, common in the USA)
ARJ - (Compressed file, common in the USA)
DRV - (Device driver)
BIN - (Common boot sector image file)
SCR - (Microsoft screen saver)

Common Symptoms Of Virus Infection

Computer does not boot.
Computer hard drive space is reduced.
Applications will not load.
An application takes longer to load than normal time period.
Hard dive activity increases especially when nothing is being done on the computer.
An anti virus software message appears.
The number of hard drive bad sectors steadily increases.
Unusual graphics or messages appear on the screen
Files are missing (deleted)
A message appears that hard drive cannot be detected or recognized.
Strange sounds come from the computer.
Some viruses take control of the keyboard and occasionally substitute a neighboring key for the one actually pressed. Another virus "swallows" key presses so that nothing appears on the screen.
Also interesting are system time effects. Clocks going backwards are especially frightening for workers who cannot wait to go home. More seriously though, this type of virus can cause chaos for programs which depend on the system time or date.
Some viruses can cost the user dearly by dialing out on his modem. We do not know of one which dials premium telephone numbers but no doubt we shall see one soon. One particularly malicious virus dials 911 (the emergency number in the USA) and takes up the valuable time of the emergency services.