What is a Computer Virus?

A computer virus is a self-replicating program that destroys other system programs like .EXE files and .COM files. When an infected program is run, the hidden virus is activated in memory and transfers the virus code to another executable program. Computer virus performs two basic functions: (1) it copies onto itself previously-uninfected programs or files and (2) it executes whatever the VIRUS author included in it. These instructions can do anything like displaying a message of thanks or peace, erasing Viruses typically infect a system through the sharing of transportable media, such as an infected diskette or through communication links such as local area networks or programs downloaded from remote system via a modem.

Types of Viruses

1. File Virus

The FILE VIRUSES attach themselves to executable files (i.e. COMMAND.COM and WS. EXE). The virus attaches its code at the end of the original program file, modifying certain portions of the file. That when the program is run, the virus code is loaded first. The virus then modifies the system so that whenever an operation involving files is made, such as doing a DIR or running another program, the virus gets a chance to attach itself to other files. After setting itself up, the virus then runs the original program.

2. Boot Viruses

The BOOT VIRUSES target the boot sector of the disk, such that when that disk is used for booting up the computer, the virus loads itself to the memory, and is ready to attack the other disks whenever they are accessed for run. There are some boot viruses that infect the hard drive’s Master Boot Record that contains information on the disk’s partition; if the virus somehow damages this information, the files on the drive will be useless or inaccessible. Early made boot and file viruses can be easily distinguished. Today, since there are various virus programmer and a numerous amount of viruses, the art of MULTIPARTITE infection is made. That is, the virus which was originally attached to a file may infect boot sectors, and a virus which was originally designed to attack a file, now attacks the boot sectors.

Here are some visible effects of commonly caught viruses.

1.Falling tears - cause all the characters cascade on the top of the screen to fall to the bottom in a pile.

2. Black Hole - produces a dark spot in Israeli in one comer of the screen and performs meaningless operations.

3. Stoned/Marijuana- displays the message, ” Your PC is now Stoned! Legal Marijuana.”

4. Bouncing Ball / Ping Pong or Italian - causes a character to dislodge itself from its original location and bounces around the screen.

5. Pakistani / Brain - replaces your volume label to (C) Brain.

6. Israeli / Friday the 13th / Jerusalem B - executable files disappear, EXE files get reloaded until they are too large to execute or fit into the memory.

7 Lehigh - changes size, data and time of COMMAND .COM, destroys all system data, activates after four infections or altering stored data, destroying Fats (file allocation table) and changing the number of bytes of all executable files causing the program to work improperly.

Hints for a Computer Virus:

1. The system hangs when you switch it on.
2. Some unwanted characters are displayed on the screen when you issue a DIR command.
3. The number of bytes of the COMMAND.COM increases.
4. Some system programs doesn’t work properly or not working at all.
5. Programs take longer than normal to load.
6. Some data files or directories of unknown origin appear.
7. Programs access several disk drives where they didn’t before.
8. Programs access disks at unusual times or with increased frequency.
9. Available disk space decreases rapidly.
10. The amount of available RAM suddenly or steadily decreases.
11. Memory maps reveal new memory-resident programs of unknown origin.
12. Normally well-behaved programs crash without reason.
13. Files mysteriously disappear.
14. Names, extensions, date attributes or data change on files or by using directories that have not been modified.

Other Anti-Virus Techniques:

Use Virus Detection Software. There are several virus detection software’s. Follow the criteria in choosing this software.
Programs and all things needed to execute must not be stored on the computers.
Program must load and execute through floppy disks.
It must not remain resident in memory while other programs are running
The program must be capable of checking all executable files.
It must check executables for changes using cyclic checks redundancy
It must never write directly to or modify the target files, disk boot sectors, partition or file allocation.
Program must provide a usage log that attacks file changes and program use.
.Change File Attributes. The DOS files have the following attributes archive, hidden, read-only and system:
Changing a file’s attribute to read-only can place in the path of roque software.

Steps in Removing a Virus on Drives A and B:

Produce two (2) copies of SCAN disk utility. One (1) for 5.25 “floppy disk and one for 3.5 disk.
To scan drive A. Boot from a clean, uninfected DOS.*Be sure that your DOS diskette is write-protected.
Gently remove the DOS diskette and insert the 3.5 “SCAN disk utility in drive B. At A:\> prompt, type B: followed by ENTER to change to drive B: At B:\> prompt, type SCAN A:/nomem/clean.
To scan drive B. Boot from a clean, uninfected DOS.*Be sure that your DOS diskette is write-protected.
Gently remove the DOS diskette and insert the 5.25 “SCAN disk utility in drive A..

Steps in Removing a Virus on Hard Disk1

Boot from a clean, uninfected DOS.*Be sure that your DOS diskette is write-protected.
Gently remove the DOS diskette and insert the SCAN disk utility.3. At the A:\> prompt, type SCAN C:\NOMEM\CLEAN*0nly the viruses found in the virus list can be removed by SCAN disk utility.
There are some virus infected files which cannot be easily removed by SCAN. However, there are other Virus Removal Kits that can be of use like CPAV, VIR-X, NAV.