1. Boot sector viruses: It infects the storage media on which OS is stored (e.g., floppy diskettes and hard drives) and which is used to start the computer system. The entire data/programs are stored on the floppy disks and hard drives in smaller sections called sectors. The first sector is called the BOOT and it carries the master boot record (MBR). MBR's function is to read and load OS, that is it enables computer system to start through OS. Hence, if a virus attacks an MBR or infects the boot record of a disk, such floppy disk infects victim's hard drive. when he/she reboots the system while the infected disk is in the drive. Once the victim's hard drive is infected all the floppy diskettes that are being used in the system will be infected. Boot sector viruses often spread to other systems where shared infected disks and pirated software(s) are used.

2. Program viruses: These viruses become active when the program file is executed. Once these program files get infected the virus makes copies of itself and infects the other programs on the computer system.

3. Multipartite viruses: It is a hybrid of a boot sector and program viruses. It infects program files along with the boot record when the infected program is active. When the victim starts the computer system next time, it will infect the local drive and other programs on the victim computer system.

4. Stealth viruses: It camouflages and/of masks itself and so detecting this type of virus is very difficult. It can disguise itself such a way that antivirus software also cannot detect it thereby preventing spreading into the computer system. It alters its file size and conceals itself in the computer memory to remain in the system undetected. The first computer virus, named as Brain, was a stealth virus.A good antivirus detects a stealth virus lurking on the victim's system by checking the areas the virus must have infected by leaving evidence in memory.

5. Polymorphic viruses: It acts like a "chameleon" that changes its virus signature (i.e., binary pattern) every time it spreads through the system (i.e., multiplies and infects a new file). Hence, it is always difficult to detect polymorphic virus with the help of an antivirus program. Polymorphic generators are the routines (i.e., small programs) that can be linked with the existing viruses. These generators are not viruses but the purpose of these generators is to hide actual viruses under the cloak of polymorphism.

6. Macroviruses: Many applications, such as Microsoft Word and Microsoft Excel, support MACRO (i.e., macrolanguages). These macros are programmed as a macroembedded in a document. Once a macrovirus gets onto a victim's computer then every document he/she produces will become infected. This type of virus is relatively new and may get slipped by the antivirus software if the use does not have the most recent version installed on his/her system.

7. Active X and Java Control: All the web browsers have settings about Active X and Java Controls. Little awareness is needed about managing and controlling these settings of a web browser to prohibit and allow certain functions to work-such as enabling or disabling pop-ups, downloading files and sound - which invites the threats for the computer system being targeted by unwanted software(s) floating in cyberspace.