I also use Avast, but Microsoft supplies a free antivirus program called Microsoft Security Essentials (which you can download for free), and with new computers includes Windows Defender with the operating system. Here is the part I don't understand. My new computer has a free trial of Mcafee, and while that is in force I can't activate Windows Defender which is already on the computer! It's a whole process (which I haven't figured out yet) to deactivate Mcafee and start up Windows Defender. Incredibly annoying. Meanwhile, I said screw it and have downloaded Avast free version.
Avast will try to get you to upgrade to a more advanced paid version, but as far as I know the free version is more than adequate. Throw Malwarebytes and/or Superantispyware on top and you are all set.