After trying a number of anti-virus', I've settled on Bit Defender Free Edition. It's unobtrusive and hasn't affected Firefox or Internet Explorer in any way as others have. It's also well ranked. I'm not sure how good it is as I haven't had any malware yet. I also use Malwarebytes and SuperAntispyware.
http://www.av-test.org/no_cache/en/t...t_no%5D=133194