I have run Avast on a 64bit win 7 with no problems. Currently running windows 8.1.1 and again sweet as a nut. However, I do not have any other security type programmes running or even installed which can sometimes cause a low level conflict. Mind you still virus free since Avast 4.0 
As this is a fresh system then as long as there is nothing untoward with it Avast should perform. But computers are like teenagers totally unpredictable