If your customer want's you to spend hours educating them on the
deeper levels of how a computer works, how they are getting infected (or PUPs)
and what they can do to avoid it... then book an hour or two worth of time
and teach them at your hourly rate.
When your car dies, and you take it to the garage... it's fair to ask what was wrong
and what they did to fix it. It's not reasonable to have a mechanic teach you to
the level that you understand the mechanics of what happened, why and then
justify their repair. No mechanic in the world is going to entertain that. They might
have a little back and forth with someone who seems reasonably knowledgeable
but there is a difference.
They don't understand why they get this crapware. No antivirus/malware/whatever
is a 100% catch all. Things slip through, customers install toolbars and whatever else
because they keep clicking next without looking to see what they're agreeing too. They
don't look for this stuff, and often times it's not painfully easy to spot anymore either.
Wanting to know why Avast didn't remove your coupon toolbar, and causing a huge ruckus
over it is a waste of time. If this is the way that client is with everything, do yourself a favor
and fire that client. In the time you waste with them, you'll be able to service a few other
better customers. If they're willing to listen to reason, educate them on how most PUP's
find their way into the computer, and show them a few of the many available quality tools
that easily and quickly remove them.
Needing a tool in addition to avast isn't mind blowing. Carpenters don't walk around with only
a hammer in their toolbelts.