Avast 4.2 - Error In Detection In .VHD File

I've been testing AVAST 4.2 fairly rigorously on both VISTA X-64 and Windows 7 7100 X-64.

Straight from the MS site I downloaded the .VHD Virtual disks (XP compatability mode)

AVAST reports that the .VHD file contains a "COMPRESSION BOMB". Now this doesn't actually tell me what this is -- but the fact that it reports something with the word BOMB in it means it can't be good.

Now as this VHD file was downloaded from the official Microsoft site I assume it HAS to be OK (or does it - I would imagine MS regularly checks its own websites - there's plenty of people out there who'd jump at a chance of attacking a MS site if they could).

I'm going to download the XP comparability mode again and have another scan.

VISTA again reports the file as CLEAN) so I think there still might be a couple of things AVAST need to fix in Windows 7 when reading compressed or "archive" type files.