The problem is that Qt itself does not support XP (even service pack 3). calibre includes everything it needs to run (no separate Qt install is needed).
Until/unless the Qt developers decide to fix Qt on WinXP, nothing can or will be done. I predict they will do so just as soon as Microsoft re-enables support for WinXP.
For more details, see the thread that exhaustively discussed the issue:
https://www.mobileread.com/forums/sho...d.php?t=245069
If you are willing to run from source and try it anyway, instructions are here:
https://www.mobileread.com/forums/sho...5&postcount=61
(edit this file:
https://github.com/kovidgoyal/calibr...i2/__init__.py)
Alternatively, depending on the fix you need, see this:
https://www.mobileread.com/forums/sho...34#post2976934