Is there any information in the report file, C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\report it will be QuickScan.txt or something like that (I have changed the file in my report settings) ?
Once you run the Quick Scan at the bottom of the scan info is a Show report file, that should show a text file with the data (the bottom of the file shows the latest scan data).
Whilst this may not reveal the info I'm after, you may have to modify the content of the report file (Quick Scan, Settings, Report file, Reported items) and check the Soft errors and Skipped items. This could greatly increase the size of the report file, but may help to get to the bottom of what can't be scanned and once we do that you can reverse the report file settings.