OK then have you checked what file list for those that aren't defragged (I don't believe there will be any in the list). Also if you run the analysis again I guess you will find you don't need to defrag and that there is 0% fragmentation.
I bet you that it is the Master File Table which is fragmented and windows can't deal with that or it would screw up the very table that records the location of your files on your hard disk. I guess that you have finally bumped into the problem area where the MFT is fragmented and as far as I'm aware the XP defrag can do nothing about that so it reports it.
Yes I get it, is it a problem, no I don't believe so, is it related to avast 5.0 I can't see how it can be. I can't recall when this started, but I believe it pre dates 5.0.
See images, click to expand.
The first shows that after analysis and before a defrag my MFT file is in 2 fragments. It also shows that there are no fragmented files in the list as the MFT isn't classed as a file in the same way that folders can become fragmented but they too aren't listed.
The second shows that notice (not error) you are talking about and on clicking the View Report matches the first image as far as the MFT is concerned yet there are no files in the list. The problem is that XP's defrag isn't too clever as it is effectively reporting what is not a file, or it would be listed in the fragmented files list, nor it seems can it defrag the MFT.