One of my acquaintances owns an HP Pavilion HDX9200 Entertainment Notebook which originally came with Windows Vista, and on which I installed Xubuntu 16.04 some time ago.

Recently, after Xubuntu installed updates that included a new Linux kernel, the computer refused to boot. Instead, it displayed some ACPI error and locked up.

Since I couldn't really figure out what was wrong, I decided to do a fresh reinstall. However, the Xubuntu 16.04.3 DVD didn't boot either, so I tried with the initial Xubuntu 16.04 DVD—which went OK.

When I subsequently updated the system and attempted to reboot, the same ACPI error happened and the computer locked up again. Apparently, HP did something strange to ACPI on some of its computers, including this Pavilion notebook, and newer Linux kernels cannot handle it. (Indeed, other Linux distros using a recent kernel won't boot either—including SystemRescueCD. Some reports about similar problems can be found on the net—e.g., HP Pavilion 15-002sp hangs on linux mint logo with any kernel about 4.4 LTS.)

It looks like this problem cannot be solved, so I decided to install the latest kernel that I knew that worked (I think that was 4.4.0.66 or some such), and to prevent newer kernels from getting installed with updates (by removing a controlling linux-generic package—though I would have to look up exactly which one). Even though the computer will, consequently, no longer receive kernel updates, at least it will continue to function.

You could always try a later branch supported by Ubuntu (e.g. 4.13).

Given the rate of kernel updates currently coming through the pipe due to Meltdown and Spectre, there may be a security issue if you leave the machine's kernel at a fixed version.

I'm not sure how Ubuntu works, but in Mint the installation of a new kernel doesn't currently automatically remove old versions, and thus if a newer kernel doesn't work, you can revert to an older one through the grub menu at startup. This saves having to do a fresh reinstall as you mentioned. Is this not the case with your system?

Actually, that's one of the things that I tried. I installed the Ubuntu HWE ("HardWare Enablement") kernel stack, which installed a 4.13 kernel. That didn't help, however.

Yes, I get that. If this were my own computer, I would want to report this issue and provide any further details that could help resolve it. However, I don't want to leave the owner without her computer for much longer. Some time after Ubuntu 18.04 comes out, I'll ask her if she will let me try and boot the installer on the computer, to see if the problem still occurs.

Yes, that's how Ubuntu works, too. It's how I got the HWE kernel stack installed in the first place.

Well, I made a long story short, in that I had done a whole lot of experimenting to see if I could do anything about the problem. I finally decided to do the reinstall because I wasn't sure if I might have messed up something badly, and I wanted to return the computer to its owner in a clean working state.