Next time, a similar situation occurred right after I logged in. Later, I could log in the safe mode and tried to check the disk. It looked flawless but the errors seemed to get ever more lethal. Different kinds of system restore were running and the worst point occurred when the computer was giving this scary blue screen message right after the startup:
STOP: C0000221 unknown hard errorThe internet was full of people who couldn't get rid of this bug even after they sacrificed all of their data and tried to reinstall the operating system! ;-) Some web sources indicated that the error means that the file, ntdll.dll, couldn't be loaded. But several discussion forums also hinted that people could get rid of that error by disconnecting their DVD burners or even by adjusting cables that go to the DVD. Again: connectors.
Systemroot system32 ntdll.dll
I opened the box, played with the cables a little bit, turned it on, played with a Packard Bell system restore utility - a different one that suddenly occurred instead of the blue screen above. It got fully cured but I can't be certain what was the reason behind all these annoyances. Because the hard disk seems to be OK and there is no obvious reason why the files should have been getting increasingly corrupt, my guess is that all the problems could have been caused by a wrong contact in the DVD connector even though the DVD is not used at all. Bizarre.
The message for all the people who got here through search engines: if you experience the problem, just turn your PC off, try to disconnect all the connectors from your CD/DVD unit. If the PC boots up, you more or less know what the problem is. Maybe if you connect the CD/DVD unit properly, everything will work just fine.
Suddenly, blue screens started again. So I disconnected the DVD completely and had the unfortunate opportunity to falsify the hypothesis above that the DVD burner was the reason. Before I went to bed, the computer just didn't work, so I suspended this posting because it's just painful to have a celebrating text if the PC doesn't work.
In the morning, I encountered blue screens with all kinds of error messages at random moments of time. It was easier to log in in the safe mode but it was crashing, too. I reported the data about the crash and Microsoft returned antivirus software (plus a clearly irrelevant program, Steam) as the cause. So I uninstalled the antivirus. Crashes continued.
Finally I decided to remove the 512 MB RAM that I added a month ago and that has been working for quite some time. And the PC works - two restarts and one hour without problems. I am curious whether they will return me the money for that memory chip. The memory seems to be a plausible explanation because the computer could have been crashing every time it tried to do something with the faulty memory chip (or its part) which would explain why the crashes were so chaotic.
As soon as the blue screens return, I will again suspend this posting. ;-)
Later, it was confirmed that the memory chip was faulty. I am waiting for a replacement. The PC works.
Update I: later, they gave me a new memory chip and it works, too.
Update II: One year later, I had a lot of blue screens again. I was blaming drivers, software, memory chips, power supply, and other things. Finally, when I removed the dust and sweepings from the CPU fan, the temperature, as measured by SpeedFan, decreased by a whopping 10 degrees Celsius and the hlue screens went away.