1)Microsoft Security Essentials
2)Avira personal edition for home use
These are two of the best free antivirus (Microsoft Security Essentials is anti-malware also) softwares. Moreover they don't slow down your system at all. I have personally used both for more than two years tested them on more than 200( Yes Two Hundred!) known & unknown infectious files & never had a problem!
Though the old saying is always true...
"A Computer Is As Safe As The User Is..."