My Acer Revo came with Avast and Win7 64-bit. It has practically nothing on it apart from Firefox and VLC. After the last Avast update I noticed that practically all video was stuttering. Checked the process graph in TaskManager, and "something" was using 25-30% of the CPU when no interactive programs were operating, and Avast was not scanning.
Experimentally removed Avast. Idling CPU use dropped to 1-2%.
Whilst it may be true to say that it requires two participants for a conflict, in this case the other guilty party may well be the O/S, and it is rather easier to swap AVs than O/Ss.