Check out my signature and you will see I use ad-aware (free), so yes there is no conflict. ad-aware isn't 'spyware' but anti-adware/spyware.
Assuming that you installed avast! in the default folder, this is the path needed to scan downloads, C:\Program Files\Alwil Software\Avast4\ashQuick.exe. This will scan zipped files, but not password protected or encrypted files.
The avast help file, right click the avast icon, select Start avast! Antivirus, Menu, Help or Windows Start button, All Programs, avast Antivirus, Help.
Browse the forums, especially the sticky topics at the top of each of the forums, not to mention the avast help file. They provide a wealth of information to help you get the best from avast.