is avast 8 free antivirus protection and detection is rate is high than any other antivirus 
There is no antivirus that has 100% detection or top notch AV product because of 100000 + viruses everyday...Every AV is just statistically effective..but there are even crappy Antiviruses out there that claim to detect everything like netprotecter and quickheal but in reality they are the worst AV's

I have been using and recommending avast to my clients for like 2 years and none have been unintentionally infected yet.

Your best antivirus is your brian that is self caution on the internet and when exchanging USB sticks.. and by the way some of the improvements that are coming along in some months is surely going to be effective.
Anyway for your curiosity:
http://www.av-test.org/en/tests/home-user/windows-8/janfeb-2013/http://forum.avast.com/index.php?topic=127472.msg953929#msg953929Still have a doubt

But remember you can still get infected with viruses if you are going to mess with your computer and dont use "self caution" its always handy to have layered security. eg: Avast free + COMODO firewall + Malwarebytes free.

Detection rates change everyday..what's important how well avast works and protects you.

If it works for you,its best for you.
Avast is almost near to have 200 million users around the world,cant go wrong with that big user base

Any further help will be provided,if you create a new topic here:
http://forum.avast.com/index.php?board=2.0