Hi tbint,
In my experience Avast able to eliminate some infections, but... is more slanted towards the Prevention rather than the Removal of Viruses & Infections.
You seem to know enough to begin looking for what you need, but to help you or someone else looking at this post...
There are some very good pen drive (or thumb drive) portable tools available, (also a great number of bootable cd/dvd isos on the net) the majority are free; that enable you to remove "infections" after booting into safe mode.
After that you can install Avast & run a boot time scan which should in effect tidy up any loose ends.
To address some of the more garden varieties of infections:
1/ Boot into Safe Mode with networking by pressing f8 repeatedly after you see the manufacturers screen or logo.
2/ Run a virus or malware scanner, & if possible update them online after installation but before running the scanning process.
3/ Reboot & install Avast Free version a schedule a boot time scan to tidy up any loose ends.
There are always exceptions that will hijack the safe mode process, but that is when you either need to take your system to a professional virus remover or get googling from a clean system.
thanks.