On my own pc I dont run any anti virus app at all, but do online scans every now and then.
But on the laptop that the whole family shares I have AVG installed. As Andavari said it is another thing when you dont have control over what others do with it.
To start with, I have 8gb ram..so I have to use 64 bit so it all get used.
Also, I have never had problems, at least with my rig, with drivers or apps not working on my comp, so I am happy with my 64 bit