I tried various stuff but none of them worked. Like RuFus, ISO2USB. RMprepusb tells me in the description that for Windows XP installation I also need to modify some files so it can't directly make it to boot. Any ideas what those files are?
WinToFlashLite on the other hand ( I tried it on Windows 10 ) fails in the process telling me that it can't lock my usb flash device (I tried closing all running programs and even un-mounting the usb but without result).
I also tried following the command suggested in this question but without luck too.
Another thing I did was simply copying over the ISO data over my USB in the chance that somehow my PC-BIOS will be able to boot it up (using linux 'dd' command).
I'm really giving it up right now. I also always wondered why there isn't some unified way of booting El Torito ISO images without burning them to disks (but maybe I'll make another question about that)?