I have installed Avast free then my PC is slowed down so much at boot with a black screen after Welcome screen.
I have run into this problem many times and many years ago (from 2013). Every time I re installed my Windows 7 and then installed Avast, it happened. By investigating the boot procedure (using Windows Performance Tools Kit from Microsoft), I'm sure it's caused by Avast. To make sure this is true, I re-installed my Windows 7 (format the whole HDD), then get updates from Microsoft, installed all necessary softwares. Everything is fine. Just right after I installed Avast, it happens.
Watch the video carefully:
https://www.youtube.com/watch?v=TAPfVzApI3gAll settings are default. I just install File system shield, Behavior shield and Web shield.
Feel free to get more information from me.