Sorry, but you don't want to hear the answers... which I gave you in the first post. Therefore, you shouldn't be surprised by what you get, especially on a public forum where most answers come from avast! users.
I installed version 1203 on WinXP x64 - and no, the sandbox doesn't work.
Yes, I get the AutoSandbox popup - but after choosing "Open in sandbox", the test application runs normally - the registry key, and also the file in the root of the C: drive, is created.
As for 32bit XP, there's no need to remove any functionality from there, because it does work, via hooking. However, hooking is not possible on XP x64 because of PatchGuard, and XP x64 doesn't have the necessary (additional) API to use instead of hooking, because it wasn't added before Vista.
Edit: as for the "young" company... really not sure how you came to that conclusion.