lyndal, I've been using Microsoft Security Essentials for about 6 months now and I like it a lot. Got rid of AVG as it was getting too intrusive and seemed to be running constantly. MSE does not rank as the best free anti-virus out there, but if you practice safe computing, it's fine.
I believe that last comparison I read in one of my computer magazines said that of the free ones, Avast was the best. I know at the computer repair shop where my son is doing an internship, they use Avast and Malwarebytes. The owner said that works just as well as Norton...
EDIT: An interesting article by Ed Botts at ZDNet...
Free Antivirus Outperformed Paid