I'm far to be an expert, but I can't help but wonder whether Windows 8 could be the "culprit" for all this mess? I mean, it's supposed to be so different compared to XP and 7 (or is it just it's GUI that's different?)
Windows 8 is a major update, Win 7 not so much, Vista was a major update, XP was minor in comparison, Win 2000 major, so... Historically this is the release pattern Microsoft has followed.
XP is getting long in the tooth, as the basic os really has not changed since release in 2000, support for SP has ceased, and as time progresses, Microsoft will support it less and less. Microsoft will not support this os after April 8. 2014.
For example, support for SATA hard drives was never included in SP3.
http://www.microsoft.com/en-us/windows/endofsupport.aspxThis means all security updates will end, and as the cutoff date approaches, less and less work will be done to update it.
So yes, Win 8 is the focus right now, and XP is looking at diminishing returns. Where Avast! devs come in, is the work necessary to work with an older os and also work with the newest. Twelve years is a long time to support any os, yet Avast! is still supporting Win 2000. So they are choosing to support quite a few os's from Microsoft.
EDIT: As this is a little off-topic, I will leave this alone and say no more.