I concur with the comments/analysis made by DavidR and warlock on the first page of this thread... there are some special files that, for your system's protection, are "protected" from defragmentation.
The most common are related to the Master File Table
C:\$MFT
C:\$MFT::$BITMAP
What avast5! probably did, via its implementation of the "persistant cache" feature, was activate/create the user journal:
C:\$Extend\$UsnJrnl:$J:$DATA
which grows rapidly, and appears to be highly fragmented.
I do not consider any of these to be flaws in avast.