Well, in fact, besides the OS itself, and TeamViewer, which I consider completely innocent in this regard, the Bitdefender was the only similarity between the machines, which suddenly started to exhibit almost identical symptoms. So it should have been evident from the beginning. I just hesitated too long...

But I can tell you, installing and then properly removing different AV products is a real challenge, especially time-wise. But that would deserve a stand-alone thread.

Czech Republic. Cursing someone is quite rare here nowadays. But one never knows.  ;)

Thanks to all.

Let me summarize in brief:

I suppose that any hardware cause is off now. Temperatures are okay, cable replaced, the disks repeatedly tested on other machines, some replaced too (with a cloned contents).

As I mentioned LogMeIn, please forget it. I meant TeamViewer, sorry, I mixed the two.

And as for the PerfDisk warning: it appeared only on one machine. On others there were different problems reported, like unreachable paging file, and so on.

As for the HD Sentinel, I don't have anything like this in all the registry.

Now, some news: On one PC I replaced the Bitdefender with a trial version of Malwarebytes. The behavior changed dramatically: now it doesn't report anything, just crashes to hard reboot. The reboot always fails, as no HD is found, even in BIOS. Reset doesn't help, after switching off the power and restarting the system starts normally. Other change is that it now happens every couple of minutes regardless of being idle or not.

So, my question is: Could there be a virus so powerful that would be capable to disable the disk controller? I still think of some software collisions, but...

I could be cursed as well. Believe it or not, but since yesterday the only working machine in the house (notebook with Vista), shows just a pitch black screen. It's working normally, I can reach it using the TeamViewer, just its display died. Funny.