avast is compatible with vista, your problem is point 1, Defender blocking ashDisp.exe the avast icon and interface. There has to be a way to have Defender allow ashDisp.exe, that is crazy if it doesn’t. I have never used defender so I’m not speaking from experience here.
If you can’t change it, delete the entry for ashDisp.exe in defender and I would hope that Defender will ask permission before blocking. If it doesn’t ask but blocks automatically I suggest you check its settings and see if you can’t change it from automatically block to ask.
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.