Thanks for your reply.
Of course I looked already into the event viewer. The only suspect messages I found there, related to HD, were repeated warnings from PerfDisk: Unable to read the disk performance information from the system (Event ID: 2001). After a series of these follow assorted crashes of random apps and disk-related errors.
These errors occur solely after some hours of inactivity. So far they never appeared while working on the machines. Even the machine left idle for a couple of days doesn't crash every night, though. So far I was unable to track down any pattern.
As this happened just recently, and as the symptoms are always the same (I mean the same files, disabling USB and network inteface), I don't believe it is hardware related. On one of the affected PC I replaced successively motherboard, power supply, cables, RAM, and finally the hard disk. So, just the CPU remained, but the results are the same. Hard to believe that CPU causes this, and on other machines almost simultaneously, too.
As for the system settings, I've experimented with tweaking things like LargeSystemCache values, as suggested throughout some forums, but without any change whatsoever.
Well, this is going to take some time. At the moment I'm experimenting with disabling automatic updates. So far, so good, but it's too early. Next, the substitution of the antivirus will follow. I'll post the result after a few days of experimenting here.
Would be grateful for any idea.