You're right. The 790 has UEFI but not Secure Boot. I should mark this topic as solved, since the original issue is fixed, and I have a bootable USB stick, though I think there must be a deficiency in the Dell OS Recovery Tool, since it did not create it directly. One question: Since the case has a Product Key sticker I had not thought the key might be embedded in BIOS. If the key is not 32-bit/64-bit critical, does this mean I could install either, including the copy downloaded from Dell for the 7010?  
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.
Dell OS Recovery Tool USB not bootable (Optiplex 790)