I don’t believe Avast 5 is actively supported, yes virus definitions should still work, but there aren’t going to be any program updates. What I don’t know is if the ‘no requirement to register’ function would be supported Or if it would update automatically after a period of time after it had expired.
It is tough to say one way or another as most users on the forums are using later versions of avast. Your win7 64bit system should be able to run the latest avast version.