well as far as I know this is just a log file, meant to refer to to get information, but in no case used by Avast to operate anything. It just contains the persistence cache log, therefor is seen by CCleaner as directly relating the content of your hard disk. Deleting it won't change the behavior of anything, won't reset anything...hey, it's not a setting file (aka *.ini), and again, it's the only log file that's not protected by the self defense module explaining how CCleaner can delete it. The persistence cache files themselves won't be affected obviously. As to the 28 MB size, yes, the longer it's been used, the bigger it becomes. Same here on my system.
ps: now I'm ready to hear from an Avast dev if this file has any other impact, I can be wrong of course.
edit: hmm David, the usntrdat.log doesn't exist
>>> you might have been thinking about the actual
persistence cache dat files. Noone has ever said that those would get deleted
...which would indeed be completely stupid.
edit: just seeing now as expected in the selfdef.log file that access was denied to the other log files.
edit: other log files get actually deleted (not all, two are protected), seeing it now. I most likely deleted them in a previous run of CCleaner without looking at the result pane, and as they got re-created, they're targeted too >>> screen shot.