I have personally used AVG Free for a good few years now, and find this as one of the best free options available on the market.
The only problem I have experienced is that every time they upgrade to a new version of the sw (e.g from 7.5 to 8.0) they try to "fool" the AVG Free users to upgrade to the paid version which I think is a bit cheeky. I however then simply go to the Avg Free website and download the new version and viola I am secure for another few Months.
When installed and configured the system require very little care, and depending on your settings it will update daily and also do a proper virus scan on a preset time of day.
For more info visit the AVG Free website on http://free.avg.com/
A review can be found on: http://tinyurl.com/czbmx
UPDATE: Version 8.5 of AVG Free is now released and can be downloaded from: http://tinyurl.com/3foztc