Dell OS Recovery Tool USB not bootable (Optiplex 790)

I have an Optiplex 790 on which I am trying to re-install the original operating system using the Dell Operating System Recovery Tool. When I enter the Service Tag, the tool identifies the OS as Windows 7 Pro 32-bit, and creates the USB stick.

The instructions for using the recovery USB stick say (in the F12 one-time-boot menu) "On the boot menu, under UEFI boot, select your USB drive". I get no UEFI boot option (BIOS boot config set to UEFI), however when I try a recovery USB I created at the same time for an Optiplex 7010 which according to the Recovery Tool had Windows 7 Pro 64-bit (though the label on the case says Windows 8 Pro) I see under UEFI boot "UEFI: INT13 (USB, 0x80)". I thought the problem must be the 32-bit OS, but the USB will not boot even if I set the BIOS to 'Legacy boot'. I tried using a different USB stick, but the result was the same.

I'd be quite happy to install 64-bit Windows, but 32-bit is the only option I get when I enter the Service Tag, and I am assuming I will only get an 'activated' OS if I stick with the original, as specified by the recovery tool.

Can anyone suggest a solution?

Thanks,

Steve.