system
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Most of heavily infected unusable machines I have seen had multiple AVs installed. Guess what - none of those AVs worked properly. Installing multiple realtime AVs makes you LESS safe, NOT more.
Answer to your problem is pro-active protection - sandboxing, LUA, SRP/AppLocker, HIPS, firewalls, behaviour blockers, … (choose your poison :P) and using common sense. Definitely NOT cramming tons of AVs on one box.