The Mainboard Still is Fine
I did test the borked XP install's mainboard after all and the only annoyance was some dust.
(All the dirt on the floor came out of this machine. It's dust soaked in nicotine.)
Due to the state of the case, I decided to disassemble what was left of this old Fujitsu Siemens T-Bird. Much to my own surprise, the cust on the case didn't spread to the mainboard at all and only the connector on the GPU has accumulated some. What certainly looks much worse is the Panasonic floppy drive, as its top was exposed for a few years. The first thing that needed to be done was the removal of dust layers on and inside the fan, on the power supply and around the GPU.
The board initially refused to start consistently. After replacing the CMOS battery, POST assumed that the replacement is just as dead and only would kick off when re-inserting it for every new boot. During the successful starts, POST also hinted at a faulty graphics' card, so this part saw the same prodecure as the CMOS battery, though now the BIOS claimed that there is an issue affecting RAM.
As it turned out, the RAM slot also collected some dust over time. Once removed, I was confronted with the GPU failure again. It took me some time to notice that I didn't re-insert it properly, which may happen when working under poor light. I finally got to boot the board with just the missing floppy drive error and later checked the missing drive that, despite its rust, still works flawlessly.
XP SP0's Overzealous Product Activation
This means that nothing fundamentally has changed since it carelessly got stored in my yard for several years. The installation of this Turkish version I got off ebay yielded no errors and even another attempt at unlocking the original install via my Hyrican tower demonstrated no issues besides the borked system remaining borked.
The only logs pointing towards some kind of hardware failure date back to May, 2014 – and due to this being an OEM install, a potentially-failing CD-ROM drive cannot be responsible for XP suddenly locking itself. The last error pointed towards an internal error when XP tried to execute VSS, which also may have hinted at a possibly failing hard drive. BUT: Unlike other variants such as Retail, OEM versions only are tied to the mainboard, thus product activation, at least theoretically, should only trigger when the mainboard itself fails. Even after treating it quite poorly, the mainboard still is fully intact and the brief issues caused by the first RAM slot did not happen when I last used this old Fujitsu – and even IF theslow already would have been accumulated some dust at the time this crash occured, the system wouldn't have gotten past POST due to the first 64kB of the RAM module having been inaccessible. That doesn't qualify as a dying mainboard, though.
As much as it is quite the relief to discover that neither the HDD, nor the the initial suspect – the replaced CD-ROM drive – were responsible for this odyssey that took me way too long to navigate through properly, the only possible point-of-failure STILL is working without showing any signs of old age, which means that it must have been Windows XP itself trying to commit hara-kiri. Partially understandable, as I was 16, stuck in some voluntary work entirely unrelated to tech, more worried about my dad's rapidly worsening health, and still influenced by the fear of tech instilled by one of my uncles who only attented a crash course once in the early 90's and then began to believe he knows everyting about computers and that he is the only one qualified to even do some very minimal system administration.
Still, it kind of is difficult to me to comprehend just how fragile the original version of XP can be even when doing nothing but playing cheap games for over a decade. There also is a certain irony that the very first set of errors did not begin to spam the logs until a few days after XP SP3, which wasn't (and still isn't) installed on the rescued hard drive, went EOL.
I likely won't rebuild this old machine to resemble its original state. The case is fairly damaged and the rescued XP install now tied to the mainboard of my Hyrican tower, which now can dual-boot both Windows XP Home Edition SP2 and Windows Vista Home Premium SP2. This tower also got better specs, so tying it back to the Phoenix board with its Intel Pentium IV and 256 MB of RAM again would be a downgrade. I got another, even older machine that also requires some in-depth tests, so there still are some possibilities to use this board and even the rusty floppy drive, which will get to see some proper care now, for something else, instead.
Unlike the T-Bird, the other machine simply is too heavy to have ended up outside and came pre-installed with Windows 2000. Its only downside is the physical lock on its case for which I never received the key. Maybe I get to break a computer beyond repair after all.