First you have to have enabled the Generation of a report file for the scan. Leave the * as the file name, that is the default file name (e.g. takes the name of the scan) and will be in the default location.
The default location depends on your OS:
XP - C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\report
Vista, Win7 - C:\ProgramData\AVAST Software\Avast\report
These locations are likely to be hidden so you would need to unhide them in the explorer, Tools, Folder Options, settings.
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You can view the basic information (number of files, quantity of data scanned, duration of scan on completion of the scan anyway. Or using the avastUI, Scan Computer, Scan Logs.through the