First of all, you are in the forum for avast! for Mac OS X, but your report is about avast! for Windows. Forum about that product is here:
http://forum.avast.com/index.php?board=2.0

Second, although I am in the avast! for Mac team and I am not too familiar with the Windows product, I really very strongly believe avast! for Windows never formats any volumes. I think, you should search the culprit elsewhere.

Best regards,
Mity