People have a very general view that there are many types of computer viruses and are those programs that cause damage or make the system crash. 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

The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware, and other malicious and unwanted software), including true viruses. Viruses are sometimes confused with computer worms and Trojan horses, which are technically different. A worm can exploit security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a program that appears harmless but has a hidden agenda. Worms and Trojans, like viruses, may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging, and file sharing systems to spread.