I'm using Avast on Vista HP with no problems, so this might be a system centric problem. You might try this, open Defender,click on tools, click on options, then scroll down to the box that says "do not scan these files or locations" and add the Avast files that Defender is detecting.
Also you should be able to " always allow" items when defender alerts you.
BTW, I can see your e-mail address, you should cange that so you won't be hit with spam.