"Malware" is a catch-all term used in the computer world to define any type of unwanted program or application that installs or appears on your computer without your consent. Common types of malware include viruses, trojans and the ever-popular "spyware." Most malware is malicious--hence the name--and will actively seek to inconvenience you whenever it appears on your computer. For this reason, it is always a good idea to have several programs on your computer designed to combat malware. Many of these programs are free to download.
AVG Free Anti-Virus
AVG was one of the first high-quality freeware anti-virus programs, and the company itself, which bears the same name as its product, has been in business since 1991. The freeware version of AVG, though it does not offer as many protection options as the pay-for-use version, has full support from the company and continues to receive virus definition updates to ensure that the program can combat the newer malware threats as they appear.
Spybot Search and Destroy
"Spybot S&D," a program created by Team Spybot, hunts down and eliminates more than 10,000 different types of spyware and malware that might have found their way onto your computer. Spybot S&D also offers an immunization feature, which will prevent your computer from infection by spyware in the first place. The program rates as one of the best anti-malware programs available, according to PC World and PCMag.com.
Ad-Aware, by Lavasoft, is another anti-malware program designed to specifically combat spyware programs that may have infected your computer, though in newer versions its uses have expanded beyond just spyware. One of its most sought-after features is its ability to combat "rootkits," which are a particularly insidious brand of malware that essentially sneaks into your computer and burrows deep into your machine's core processes before activating, making them difficult to track and defend against. Ad-Aware is free for personal at-home use.
Similar to AVG Free, Avira AntiVir is a free-for-personal-use anti-virus program designed to provide protection against all types of malware that may try to infect your computer. Avira--like AVG--is free to download and is continually updated with the latest in virus definitions to keep its defenses current. The only downside to the program is the "nag" screen, which is what some users call the pop-up ad that displays every now and again. The pop-up ad appears at regular intervals encouraging you to purchase the full version of Avira. You cannot disable this ad from within Avira.