I know that it’s possible to schedule a virus scan at a given day and time using Scheduled Tasks.
By using the AshQuick.exe file that will schedule a quick scan.
However, which file should I use to schedule a Thorough scan?
ashQuick.exe is anything but quick it scans everything and as far as I’m aware it is a one setting option, scan. It isn’t designed for large scans, rather ‘quick’ scans of downloaded files, context menu file or folder scans, etc. but not system scans.
Check This out, I assume you are using avast home not pro as it has its own task scheduling for scans - HOWTO: Scheduled Automatic Scans avast! Home Version!
The major disadvantage with the task scheduler trick of using ashquick.exe is, it will scan every file of the hdd, partition, folder or file that you set it to scan; even those files not considered a potential threat, this can take a long time. It is possible to add certain folders to the exclusions list for say your media files, etc. This could leave you vulnerable if that folder got infected as it wouldn’t be scanned by the on-demand scanner as you have excluded it.
The Pro version gives a lot more flexibility and the scan would follow the settings and exclusions, etc. of the regular scan.
With the different levels of protection of and the fall back of the Standard Shield, I find the need for daily scheduled scans lessened. I prefer to do a manual scan with my regular weekly maintenance tasks.
DavidR
In fact, I do want Avast to scan all of my HDD.
So, if that’s what ashQuick.exe will do (whether quickly or slowly), that’s fine by me.
Thanks for your response. 
Then you should use the avast Simple User Interface, that give multiple options Standard or Through scan, with or without archive files. There is also the ability to scan Local Disks (whole HDD/s), Folder selection (Partition/folder/file, etc. don’t select both of these options together as you duplicate the scan) and Removable Media.
To access this, right click the avast icon, select Start avast! Antivirus.
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