One common PUP that AVAST doesn't flag up is Open Candy.
Both Malwarebytes and Spybot scans will identify it as a PUP and get rid of it. Unfortunately there are a significant number of otherwise good 'free' programs that now come bundled with it as a non-optional extra. You won't know this unless you read through the program's User Agreement conditions thoroughly and those can change even when you are updating an existing program.
I'm sure many of us don't thoroughly check an update installer's UA and so such stuff can easily get on to a computer. I suspect that is how a lot of other PUPs get installed.