I've been using it on one of my machines, dumped avg for it as I was tired of re-installing the free version when they do a major release. Windows defender (which I assume is what we are talking about here) is very lightweight and has a couple of features that I like, for one it updates itself through windows update so you have one less updater running all the time and two it has a tool called 'Software Explorer' that shows you all of your running programs, startup programs, network connected programs and winsock providers and their threat classification / software verification level. I find it helpful in collecting all of that information in one place.
As far as effectiveness I can't say from experience as I didn't get viruses before and still don't but I also have other defensive measures in place. Overall I think it's worth a try.