NZFF

It seems to be a constant battle to keep computers free from viruses and malware. My partners computer was really suffering from hang ups. For example I tried to transfer 334mb of data to a USB flash drive and it just got halfway and stopped. Same with many other programs that just seemed to half open and then give up or be painfully slow. This is despite running many different antivirus programs as well as a regular clean with Ccleaner and Malwarebytes. None of these programs indicated any issues with her PC.
In frustration I took it down to my mate at the local PC shop (he used to be an IT techie with Air NZ years ago). It took him quite some time to eradicate the many viruses and malware that was on her system.
He highly recommends 2 programs that he only uses and it was these programs that flushed out the gunk that others didn't even know was there.
They are:
Anvi Smart Defender
Kingsoft Antivirus

They are both free. The Kingsoft does not work with Win 8 as yet.
I ran them on my new system and came up with 47 infections (2 trojans) that none of my current defense picked up (must be those dodgy flight sim sites!). The kingsoft will also light up if an infected USB flash drive is inserted. Pretty good.

Freewares , paywares , and the tricky tricksters, Malwarebytes Anti-Malware is what i use, recommend by my local PC Teckie at 'Calder Electronics' and use it regular, I do recall mention of clash's in the early days with different anti virus software , generally now I follow my technician .... Or .. in-house computer scientist Greg , pretty much stops for a coffee/tea or maybe a lager every week. .... Crikey I'm so lucky !

Avast is a good one, one of the best for freeware too (Just turn off the damned annoying voiceover in options lol)

I have used Avast in the past but I think Kingsoft might just have a few more features such as USB checking.