I find Avast to be really good, even the free version is great. Avira seems to be good, but every time it does an update, the free version brings up a pop-up window that you have to dismiss, asking you to purchase the full version.
Also, the free version of Avast has real-time email scanning as your email comes in. Avira doesn't, but it will stop bad attachments when they are launched. Pick your poison.