Setting up the Z390 Dark to run Windows XP is a bit of a hassle but doable. I will here show the simplest steps with the relevant tools and files included.
1. Make sure you update bios to version 0.92. or 0.93
2.Burn the attached iso file with Nlite directly to a DVD RW disk. This version is very light for OC benching and thus dont have all the bloatware. Neither does it have any USB drivers integrated. It is a Windows XP 32 bit SP3 professional which is around 160Mb.
https://www.nliteos.com/download.html
3. Make sure to pre-format the SSD drive in Fat32 and make sure it does not have any UEFI boot sectors from other installations. This will make it less likely for the windows XP installer to fail with A5, B7 errors.
3. Make sure to connect the SSD and DVD drive to the Asmedia ports on the EVGA Z390 Dark motherboard.
4. Connect a real PS/2 keyboard to the PS/2 port. If you have both PS/2 keyboard and PS/2 mouse connect both through a PS/2 splitter to the Motherboard. The 0.92 bios directly work with the PS/2 units. I have not integrated any USB drivers in the image on purpose.
5. Boot up in Bios and set boot to legacy and enable Windows 7 installation. save bios settings and reboot. ensure the SSD drive is set as first boot drive and the DVD as second.
6. Again boot up machine into bios and then force boot to DVD.
7. Windows will start installing. Do not press F6 to install drivers. If install fail with A5 or 7B errors reboot the machine and again force to boot from DVD.
8. If step 7 failed the first time repeatedly press F7 when windows installer ask you to press F6 for driver install. This will bypass the ACPI HAL detection during install and let you proceeed with installation.
9. Windows XP installer should detect the SSD drive and i recommend you clear all potentially existing partitions so Windows XP itself will redo the partitions during install.
10. Windows installer will reboot and restart.
11. When it asks for registration key use the following key.
FY32F-XF3B6-277BF-YWBQF-GVRX3
12. windows install should finish.
What is important to note is that even if Windows XP might start there might be issues hidden. I have tried many different versions from downloads and in some cases only 1 core is visible in CPUz and actually active thus wont be usable.