Archive (zip, 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. Thorough is also by its design very thorough and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate.
The above is my own personal feeling, that would be over ridden if this your first full scan since installing avast as this would do more to confirm you system has a clean start state. It is likely to take much longer.
