The developper of security and anti-virus McAfee precise the "malware" (virus, pop-up ads, toolkit, spyware, adware, etc.) and increase recently. As you didn't notice, internet never be safe today. Pepare to protect yourself or else you will be one of the 100 millions PC infected by random virus. As I think the "malware" will increase every year by those online advertises by text picture and ads. Also, some site pay you 1 cent to see 1000 ads. Think about how much time and money you pay you see all 1000 ads? you will also count the electricity, the PC and also the internet bill. Will 1 cent pay those bills? I hope your PC will not have to slow down or else get a malware scanner to do it.

Here some suggest: all is free
Spybot Defender (Spyware detection): http://www.safer-networking.org/
Grisoft Anti-Virus: http://free.grisoft.com/doc/1
Lavasoft Ad-aware Personal: Here

With those three in your PC you are 45% protected your PC from "malware". How about 55%? That's simple, it's YOU! The best thing to protect your PC is never run a suspicious program. You are boss of your PC. If you can't control your own Pc then give him a slap! Then make him listen to your command. The worse thing will be "Format" word.

Sources: http://www.clubic.com/actualite-36341-mcafee-definitions-de-virus-doublees-en-2008.html