Use the right anti virus (with the right settings) and it doesn't slow you down or crash other software. Use the wrong anti virus and I can be a nightmare (I'm looking at you Norton).

I worked one place a few years ago that installed Norton on every machine. I caused huge hassles on our developer machines, as it would cause a huge slow down every time a new exe was created, which being developers was a dozen times an hour...

AVG has saved me from infections/trojans a handful of times in the last 10 years, all of those times when receiving files/emails from family members or colleagues. I've probably had twice as many false alarms, but at least AVG make those fairly easy to deal with.