Thank you so much for your help, AdarshN, but it does not seem to be solved as I thought.
It's back! I tried to use the command prompt and received cmd.exe crashed message, now not with the 0xc000007b error code, but still with ntdll.dll on the details.
Then, following the instructions of point 6 on the page of your link, I disabled Data Execution Prevention for cmd.exe and it stopped crashing if I call cmd.exe directly, but it crashes if I try to use a (not installed) little program from AMD for CPU ID which displays the information about the CPU on a command line window. The Update Checker issue was back too. Then, I rebooted as advised after the Data Execution Prevention settings. It did not boot. I used the Windows DVD to recover with System Restore. There were 3 updates, I don't know how because I hadn't turned them on yet because I was investigating the issue; they should be drievers. I took the most recent restore point and it booted again.
I'm lost and don't know whether the HD bad sectores were from the issue or the otther way round, so I left HDD Regenerator find any bad sectors iver night and cure them. This time there were 2. Before I'd find about 40.
As I have all my files in 2 other HDs, I didn't touch them. I formatted the HD with the OS/MBR where I place the OS and all programs, …