Well avast doesn't delete anything autonomously, avast scan and alerts to infection and you choose what action avast takes.
With the pro version of avast you can set up actions on how to deal with detections, so if you put delete as an option it could delete as you had given that as the option. However, I don't know if the anti-rootkit scan (which this was) conforms to those pre-set option in the pro version.
If you don't select delete than avast doesn't delete.
If avast detects something as infected then it will be in the avast log viewer, if it is in the anti-rootkit scan it is in the C:\Program Files\Alwil Software\Avast4\DATA\log\aswAr.log log file, view it using notepad. At the bottom of that there is a summary of the scan, files scanned, infected, etc. This is only there until the next anti-rootkit scan (8 minutes after the next boot) when the new log overwrites it.