First, download Avast normally. Once it's done, (if on Windows,) type in the search bar, "Programs" and there should be a search result about managing programs. Scroll down until you find Avast. There should be two options, uninstall, and modify. Click on Modify. A box of options should appear, and you can untick everything you don't need, while keeping Passwords ticked on. This will effectively disable the Antivirus function, while keeping the Passwords function.