someone else will have to answer the rootkit question
but did you try and schedule a boot time Avast scan
rt click the ball - update- rt click the ball schedule scan
reboot
quarantine/chest do not remove/delete
post the log
general instructions follow
The C:\Program Files\Alwil Software\Avast4\DATA\report\aswBoot.txt
provides a more user friendly summary of the boot-time scan and it should list any detections.
here are the instructions to send result to virus total shamelessly copied from DavidR
You could also check the offending/suspect file at: VirusTotal - Multi engine on-line virus scanner and report the findings here. You can't do this with the file securely in the chest, you need to extract it to a temporary (not original) location first, see below.
Create a folder called Suspect in the C:\ drive, e.g. C:\Suspect. Now exclude that folder in the Standard Shield, Customize, Advanced, Add, type (or copy and paste) C:\Suspect\* That will stop the standard shield scanning any file you put in that folder. You should now be able to export any file in the chest to this folder and upload it to VirusTotal without avast alerting.
If it is indeed a false positive, see
http://forum.avast.com/index.php?topic=34950.msg293451#msg293451, how to report it to avast! and what to do to exclude them until the problem is corrected.