I have now booted the computer from the stick and formatted the hard drive completely (formatting is in progress). Who needs Windows anymore?!?!?
I assume this will solve the problem.
I'm going to buy the computer a new SSD anyway (I don't trust a 7-year-old SSD). If the problem is hard drive related - and it looks like it to me - then I won't have a problem anymore.
Or what do you mean?