This is a Windows XP Professional SP3 ISO that has been modified to work on i486 processors. There is a German ISO and English ISO available. The CPU clock speed in Windows system properties is also incorrect, but programs like CPU-Z report the correct clock speed.
NOTE: After the installation is finished, you must copy ntoskrnl.exe from the ISO to C:\Windows\System32, or else the OS will be unable to boot.
UPDATE 5/19/2024: The English version of the ISO is now available thanks to roytam1!
Credit to Dietmar from MSFN: https://msfn.org/board/topic/186080-xp-running-on-a-486-cpu