I was using AVG for my windows but it didn't worked for me so I switched to Avast. It seems to work nice for me, and does not seem to slow anything down. I will stick to Avast until and unless it won't create problems to me.
I used AVG before but newer versions slowed my old PC a bit. I discovered AVAST and it's reliable so far. I think AVAST is the least demanding on system resources and most importantly, it's free! The only downside is that it has no boot-time scanning for 64bit systems. But who cares? I use 32bit windows anyway