There's a couple of 'bigger name' free ones out there, namely AVG and Avast. My personal opinion and experience of AVG is that it looks like it's working, when it actually isn't, despite being kept up-to-date.
So I stopped using it. (any e-mail I receive that says that it's been scanned by AVG and is certified virus-free I always scan myself!).
Avast is better... you can get it here...
http://www.avast.com/eng/avast_4_home.html
I use my ISP's own anti virus and anit spyware programme - much better than any freebie you can find and it does come as standard with the larger packages (for smaller ones there is a charge)