It you're only willing to go for free software, then I would say the only other decent one out there is
Avira. (Due to issues with recent updates in AVG I can not recommend it any more)
However, if you are willing to splash out a little then I would say go get ESET Smart Security - It is definately worth it, fairly lightweight, and depending on configuration quite unintrusive. Currently for a 1 user 1 year license it goes for £20 on amazon, with a 3 User 1 year license going for £25.