I’ve done this before multiple times it was fairly simple but I’m trying to install a new windows OS in a separate standalone Qubes, same settings through Qubes settings and I’m getting a driver not found error this time. Never happened before. Windows loads the installer then when it’s time to select the driver to install on it brings up a load installer screen and says a media driver is missing. “This could be a DVD CD blah blah. If the installation media for windows is in the dvd drive or usb you can remove it now” I’ve tried booting from a USB, booting from an iso in a Qubes same issue. Both usb and booting from a Qubes van with the iso worked before so not sure what’s causing this.
I’ve tried Windows 7 Ultimate SP1 that I had and I didn’t have any issue with it so maybe your ISO is missing some Xen drivers. Try some other ISO or maybe you can try to download Xen drivers for Windows and select them during installation: