Please post the files
c:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\log\aswBoot.log
and
c:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\report\aswBoot.txt
If on Vista and higher, they are located in
c:\ProgramData\AVAST Software\Avast\log
c:\ProgramData\AVAST Software\Avast\report
Did you get the message really just during the scan, or rather during an action on in infected file?
The second message must have been an action - it means it's impossible to remove the particular file from an archive (such as a solid RAR archive, for example).