Which free edition you would prefer? Avast, AVG and Avira all offer free editions of their respective products free of cost for home users (breaching the agreement many use those products at corporate environments as well). We have fully licensed Symantec Endpoint Protection Suit and while we started lab testing Windows 7, the package was not compatible with Windows 7. We needed to have moderate level of protections for our Windows 7 boxes and tried out all the above mentioned Antivirus products.
From the experiences till date from last four months, Avast is the winner! Starting with a clean interface and user-friendly GUI and updates are just in KBS this stood apart from AVG and Avira. AVG installation was always testing our patience when Avira has a poor appearance and heavy on the system (AVG also slows down the booting part). There are some comprehensive study made by www.brighthub.com which benchmark these products in details. If interested you can read the full article here. Though the writer finally choses Avira Antivir against other two products, we prefer the latest version of Avast against both AVG and Avira.
Hope this information is useful for people who wonder which free antivirus product they should go for. We, Windows7bugs team would ask the users to avoid visiting sites which malicious in nature (Software cracks, teaching hacking etc) and to avoid those small applications which are written for cracking BIGGER software. The less risks you take consciously make these free products provide you better protection.