The first one? Who knows... Before I had my private computer I saw my dad's Windows XP machine being filled with adware, random popups, the obligatory "your PC has been blocked by the FBI" (Polizia postale in Italian) like the ones you guys posted screenshots of before (I believe it was just a windows guide file locked to full screen), whatever crap the Softonic installer decided to put on the hard drive, and so on.
Most of those incidents happened because my dad refused to install a real antivirus while downloading crap from every website he came across.
Fortunately my uncle was always ready to format and reinstall everything, or maybe go in safe mode and check what happened.
Skipping becoming 10 years old and getting my first computer with windows 7 (which I still have), and using over and over the system recovery utility from all the noob errors I've done, the worst one I ever saw was surely the original Cryptolocker, but not on this PC:
My mom's office had a little network with two computers, and I now had enough knowledge so that everyone called me every time some piece of tech broke.
One day, I was coming home from secondary school, and she calls me, and talks about this weird window and how all* of the files got renamed; as she talked, I knew what was coming, coincidentally some days before I read an article about ransomwares, but I couldn't believe they were already in the wild...
The virus came from the secondary PC on the network, and then spread into my mom's one. It turns out, the secondary computer had the antivirus uninstalled by my dad! :Facepalm: but I still don't know if it got through initially via some email or using some exploit... (Also, my dad was 100% anti system updates, because "they make internet slower"

). From that time on, I was more careful with how the computers I came account were protected...
Now it isn't a problem anymore how many times my dad tries to open taxes.vbs
*Fortunately the database extension of the main program they used was not in the blacklist of things to encrypt, so they managed to get a copy and start clean again.