As Bluesman said this is a function of the Pro version, the free requiring interactive action.
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.
I have only ever done a through scan with archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.
I appreciate that you detected a virus on your scan so it might not seem overkill for you to have done a thorough scan, but the file type and location may still negate it a little.
What is the virus/malware name, the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ? Check the avast! Log Viewer (right click the avast icon), Warning section, this contains information on all avast detections.
It could be a new signature that has been added that now picks up something that was dormant in your system. So the information on it could help us give more advice.
