It may be nothing to worry about see below, I take it this was on an on-demand scan ?
Corrupted Archive file, this could simply mean that avast is unable to unpack it to scan the contents of the archive and assuming it is because it is corrupt. Even if it were corrupt there is nothing that a user can do to resolve any corruption, short of replacing the file.
Yes it may just be that for some reason it isn’t able to unpack the file to scan the contents.
Files that can’t be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.
I had a few reported as corrupt, but nothing I could do about them, the only way it could be a problem is if they are actually corrupt and something tries to use them, then would get an error message to that effect and I presume you aren’t getting that ?
You can check the following report file using notepad, C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\report\aswBoot.txt, this is for XP it is in a different location in Vista ProgramData some where, try a search for the aswBoot.txt file.