Scan when opening
If the box “Scan documents when opening” is checked, Microsoft Office documents and other OLE files will be checked at the moment they are opened. To determine which documents and files to check, avast! will check their actual content, not just the file extension.
You can also specify files that should be scanned based on their extension.
To scan files with a specific extension, check the relevant box and type the extension in the box provided. You can use the wildcard “?” for example, if you want all .htm and .html files to be scanned when opening them, you can enter both extensions, or just use the wildcard “ht?”. Note however, that this would result in all files with extensions that begin with “ht” being scanned e.g. files with the extension .htt.
Alternatively, you can specify that all files should be scanned, however this could slow your system down quite significantly while the scan is in progress.