Windows Defender is just fine, really you don't need a anti virus if you take the right precautions. Have the right addons on your browser like an adblock, noscript, and BetterPrivacy-signed, so you can't get some mischievous malware while browsing the internet. One of the best things you can do is change your host file to block ads on your machine all together too. If you do this it blocks ads on any program that has ads in them like Skype. More can be found here:
http://someonewhocares.org/hosts/
Don't download and run random .exe's from websites and if you want to download them in a VPS first and try them out first to see if their fine. If you are torrenting things make sure the torrent has a few comments on it first to make sure it's safe. Even with a anti virus you can still get virus and malware, your computer is only going to be as secure as long as your don't do stupid things.