I have avast free one and I'm really happy with it.
Some other free ones I've took a lot of memory to run, but this one seems pretty unobtrusive and it was easy to download.
I did a bit of research into them and Avast was rated quite highly in terms of picking up viruses, and it has found two viruses so far while I've been browsing the net. The only thing I hate is it asks me what I want to do with the virus its found, eg. move it to vault, heal it, delete etc. I dont know! lol