I'm back again, still alive, and my computers, too. It was not a discourtesy, I just wanted to test everything thoroughly before posting, and it all takes so much time...
So, my first step was a workaround: I automatically restart all the computers automatically overnight. This solved a lot of problems, but not all and I'm aware that it is not a solution. However, rebooting Windows machines often proved to be generally a good idea.
Now, I'm almost 100 % sure that the culprit was the Bitdefender Free Edition
. I experimented with some other AV solution, and ended-up at AVG Free. To make it short, since I replaced Bitdefender with AVG, the following symptoms never appeared again
- Delayed write fail error
- Disconnecting USB and network cards
- Occasional blocked access to Scheduled Tasks
- Repeatedly erased cookies
- Random crashes of browser (using Opera 12)
This all now stopped appearing. I left the automatic rebooting in effect, just to be sure. So, I'm not sure I really found the culprit, but I suspect that I did. So far, so good, and thanks to you all for your support.