Also why doesn't the separate virus cleaning tool work?
The tool is only for certain infections, not ALL infections.
why are they back?
Since I can't check on your system I have to guess. These are some possibillities: - They where not (completely) gone in the first place - Browsing behaviour - Download behaviour

My suggestion, run a boottime scan and let Avast handle it or (perhaps even better) have a look at the page in my signature and follow all steps as explained there.