Yeah, AVG is good and Avast also gets good reviews and both probably give slightly better protection than what's built in. However, Windows Defender is so unobtrusive that for me it's a no brainer. If you're going to the dark scary corners of the internet and you found that in Windows 7 AVG would pop up warning about some dodgy file very frequently it may be worth it. Personally, I prefer the extra convenience of Windows Defender over the slight security advantage.