Virus: A virus is a computer program that has been coded to replicate itself and infect a computer without the user's knowledge. A virus spreads from computer to computer by infecting network files as well as CDs, USB drives, and other forms of removable media.
The best free anti-virus is AVG anti-virus which can be found at http://free.grisoft.com/ .

Trojan horse: Trojan horse or just Trojan is a program which looks legitimate but performs illegitimate actions such as granting full access to the intruder, installing a keylogger, disabling security software and as an invisible downloader and spywares. Spybot Search and Destroy is a free program that can find and delete Trojan horses and spywares. It can be found at http://www.safer-networking.org/en/download/index.html

Adware: Adware (advertising supported software) is a software bundle which automatically downloads and displays advertisements to a computer in which it is installed. Adwares can be classified as privacy invading software. It is seen as a way to recover development costs by software developers. The best program to combat adwares is Ad-Aware 2007 and it can be downloaded at http://www.lavasoftusa.com/products/ad_aware_free.php

Rootkit: A rootkit is a program which has been coded to take complete (or root) control of the system without the user's knowledge. Rootkits take full control of the operating system and therefore cannot usually be removed by anti-virus software. Rootkits usually modify the boot sector of the operating system or disguise as drivers and load during boot up. As a result, it is usually hard to delete them. The most recommended rootkit remover is Sophos Anti-Rootkit which can be downloaded at http://www.sophos.com/products/free-tools/sophos-anti-rootkit.html