I got a Compaq Deskpro ENS/P600 (the EN SFF [for "Slim Form Factor"] series) without a hard drive and neglected in a very dusty business environment.
I gambled on a small SSD and a SATA to desktop IDE adapter, cleaned everything inside and out, and installed the SSD. The Windows 98SE install CD recognized it.
I had to spend an entire evening locating the relevant Softpaq files (only four out of a few dozen thousand with cryptic names).
Today, I installed 98SE and the Softpaqs. ("98SE and the Softpaqs" is now my Rock-n-Roll tribute band name.)
I also turned on and started updating a MeLE PCG02, a Windows 11 IOT-like fully contained desktop PC. It started downloading and installing KB5089549.
From FDISK through Win98 setup to every Softpaq installed and a fully functional desktop with working graphics and built-in speaker audio took about an hour.
Windows 11 is still installing the one single update.
Most of the time I know my right from my left. The first inside picture description was not one of those times.
I tried putting the SSD in a 2.5-inch to 3.5 inch laptop-to-desktop drive adapter, but there was simply not enough space between the drive cage and PSU for that and the IDE-to-SATA adapter as well. Not even if they were forced in enough to cause damage.
The insides, although pretty much entirely proprietary, is surprisingly accessible. The only things needing a screwdriver to remove are the expansion cards.
The most annoying proprietary problem is the CD/DVD cable: I'm pretty sure it has the right number of pins for an IDE cable, but both ends are smaller and the pins thinner and more closely spaced than standard IDE connectors.
Also, the DVD-ROM drive is the only thing with a date code I could find: It was made in 2000.