Bootable USB to install Windows on MBR HDD?

Greetings.
I'm running Windows 8.1 on an UEFI system, clean installed from my USB-key created with Rufus.
But when I try to boot with this USB-key on my older PC (which does not have UEFI), then before installing Windows it says that there is no HDD or partition Windows can be installed in? I can then boot again with USB-key select rep. PC, start cmd, change the HDD to GPT, then Windows can be installed, but then it can't boot from the HDD!!

How do I make a bootable USBkey with Windows 7 or Windows 8 that can be installed on an ordinary MBR HDD? I need to make it on my system running GPT SSD.

I have tried this:

CMD (elevated)
diskpart
list disk
select disk (selected my USB)
clean
create partition primary
active
assign
exit
and then copied my windows 8 dvd onto it.

It will boot, but Windows will only install on a GPT HDD.

What to do?

Hello Burgurne,

The tutorial below can show you how to create a bootable Windows 7 or 8 USB without UEFI.

http://www.eightforums.com/tutorials/2227-create-bootable-usb-dvd-windows-8-iso.html

Next, use Option Two to clean and convert the disk to MBR, then try installing Windows 7 or 8 normally with MBR.

Convert GPT Disk to MBR Disk - Windows 7 Help Forums

Hope this helps,
Shawn

Thank you, Shawn.

Do you know if this will work?
Backup my system SSD. Delete my system SSD. Boot from USBkey. Convert it to MBR, restore my image (Acronis backup). I know one (or more??) of the small partitions is not needed in MBR, but will it work anyway, if I tell Acronis NOT to overwrite SSD "MBR and 1. sector"?

I would definitely backup anything that you wanted to keep, but it may be best to start from scratch, then copy back any files you wanted from the backup afterwards.