If you have Genuine Windows use the built-in virus guard "Windows Defender".
If not & you need free virus guard use "Avira Free virus guard".
But it may slow down your PC little bit but when you set it's scanning settings high, it does really good job even though it affects performance.
I have used "Avast Free" before, it is fast and not much performance change but it's not good as "Avira" in virus detection.
No matter what virus guard, internet security suite, paid or free, you must customize it's settings for the optimal virus scanning otherwise virus guard won't do a thing.