Well .zip files aren't scanned by default in the standard shield as they are of negligible risk.
Archive (zip, rar, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned.
What type of scan are you using ?
If as you say you can see the file in explorer, right click on it and select 'scan select areas for viruses.' This should scan either, dick, partition, folder or file, depending on what you right click on. There is a brief pop-up to say it has been scanned and it closes, this is the avast quick scan (ashQuick.exe) there is list of files scanned displayed, etc. That is the idea of the quick scan to let you get on with whatever it was that you were doing.
If the scans finds an infected file, all hell breaks loose with an audible and visual alert and avast asks what action you want to take. So if no alert nothing has been found.