Malicious code is the general word used and virus is one type of malicious code. Worms and Trojans are different from computer virus and destructive types of worms are sometimes termed as internet types of virus. Computer viruses have made their mark on the PC's since the 90's with different types of viruses causing damage to computers.

The earliest viruses made the most impact as no anti-virus programs were available. The advent of anti-virus programs spelled a death blow to many common types of computer viruses that were written by the various tools to generate types of computer virus. Those who were inept at programming used these various tool to generate the virus. Devious mind always find new ways when they found that the anti-virus software's were able to disinfect and remove the different types of computer viruses they went on to new methods and that is how Trojans and worms came up.

There are a few main types of computer viruses and these computer virus types are classified below depending upon the infection methods:

Boot sector/Master Boot Record Computer Virus

Means of Infection

Boot sector is that area of the computer that is accessed when the computer is turned on. A boot sector virus infects this portion. Once the boot sector is infected the virus is loaded into memory when the computer is turned on. This virus then infects boot sectors on floppies or other removable media Master Boot record virus only infects the Master boot record and not the boot sector

Damage Caused

Boot sector viruses gain complete control of the master boot record or the DOS boot sector by replacing the operating system contents with that of its own. This allows the virus to spread fast and cause damage:

By gaining control of the master boot record and the DOS boot sector the boot sector viruses can sometimes hide the resources that the computer has( the floppy drive even though attached may not be present)
Some boot sector viruses contain instructions to redirect disk reads.
Some boot sector viruses move the master boot record to another location causing the system to crash when it boots up. Other boot sector viruses cause damage to the master boot record
Some boot sector viruses damage the File Allocation Table (FAT) which is the index of all the files on the drive. This causes loss of data

Removal

The best way to remove boot sector virus is to boot the computer with using a clean boot disk and then rewrite the files with the good operating system files on the infected disk. These viruses were very much prevalent in the ninety's and a host of antivirus programs are now available to detect and clean them effectively

Continue to : Different Types of Viruses

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Beware of Email Viruses in Spam - Top Tips to Stay Safe
Popular Email Viruses and the Effect They Had on the Internet
Macros: Better Applications or Worse Viruses

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Some Viruses can Crash you PC. Check out the Types of Viruses

Boot sector/Master Boot Record Computer Virus

Means of Infection

Damage Caused

Removal

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