The trick is to turn off the other PC after files are copied, and PC restarts. Of course, starting with 32-bit install media (for such an old laptop) is a must, as is setting BIOS to legacy, no UEFI etc.
The problem now is the software asks me a username and a password to access the shared folder that contains the windows file extracted from the iso.
As username I specified the one I inserted when I configured Serva (as the youtuber) but he doesn't insert any password. If I do the same I get an "0x56 incorrect password error"
I don't know where this password come from...