There are many different types of virus / programs which can harm and slowdown the performance of your computer. Some of the major ones include :-

Adware

Adware is not self replicating. It is used for an advertising display that has not been requested, its most prevalent on free or cheap web hosting sites.

Backdoor Trojan

A backdoor Trojan is not self replicating. Though it can seem legitimate, a backdoor Trojan is used for accessing a computer through the internet.

Controlled Application

Though a controlled application is legitimate, it may not be useful to many computer users. It can be removed because it may be a resource hog or because it slows down productivity.

Dialler

A dialler does not self replicate. It is used for dialling (adult) phone numbers with a premium rate.

Joke

A joke does not self replicate. It is meant to fool a user into thinking their computer has been infected. It will not harm a computer and you can safely delete it.

Junk

This is a virus code that is no longer active.

Keystroke Logger

A keystroke logger can be part of malware. It records keystrokes in order to gain private information such as credit card numbers and social security numbers.

Malware

Malware may or may not be self replicating. It depends on the particular malicious software of which it is a part. Malware is used as a general term to describe spyware or a Trojan, virus or worm.

Malicious Behaviour

This term describes the behaviour of any executable file that can intrude on, disrupt or damage computers and/or files.

MSH Virus

A shell is the text based command line interface used to communicate with a computer. This can be used instead of the Windows graphical interface. When the MSH virus is executed, it will infect other Microsoft Command Shell files.

Potentially Unwanted Application

Examples of potentially unwanted applications include adware, diallers, hacking tools and remote administration tools. Though the process may not be malicious, it can be unwanted and unnecessary.

Registry Virus

A registry virus manipulates the registry contents. A registry virus can infect the operating systems Windows 95/98/Me and Windows NT/2000/XP. The registry stores information about a computer’s application software, computer memory, hardware and system options.

Remote Administration Tool

This EXE file is not self replicating. It is used for remote access and control of computers without their owner’s permission.

Rootkit

A rootkit hides its files, data and processes. A third party can infect a computer with a rootkit.

Spyware

Spyware is a general term. Spyware forwards computer usage and user information to a third party.

Spyware Worm

A Spyware worm can self replicate. It does not need a host program. Spyware worms forward computer information to a third party. A spyware worm can infect the operating systems Windows 95/98/Me and Windows NT/2000/XP/2003. Spyware worms exploit the weaknesses of an operating system or application. They can infect other computers through email or a Windows networking APIs (Application Programming Interfaces).

Spyware Trojans

Spyware Trojans do not self replicate. Though a spyware Trojan may seem legitimate, its purpose is malicious. It forwards a computer’s information to a third party.

Trojan

A Trojan is not self replicating and can appear to be legitimate. Its purpose is to cause problems with the activity of a computer.

Virus

A virus is a self replicating EXE file. After its host program is executed, it will infect a computer. A virus can send itself through email and can infect a network by self replicating itself. A virus works by damaging computer data or disrupting computer systems.

Win32 Executable File Virus

This virus infects executable files through different ways including Outlook or other email software. Computers using a PE executable file format are vulnerable to infection by this virus. Computers vulnerable to this virus include Windows 95/98/Me, NT and other Window operating systems.

Win32 Worm

This virus infects email users and Windows networking APIs and MAPI functions. A user may be enticed into opening the infected message by creating interest in its content or importance. It can send email messages with the virus attached or it can add itself to email messages.

Worm

Worms can self replicate. It can work without a host program. It may use email and the internet to infect other computers. It does not infect files.