I have an HP tower Windows 8 64-bit PC. I have to go into the BIOS to set the option to boot from USB. It has two options. One is to boot from a USB CD or floppy (that's what it says) or from a USB hard drive. No matter what I choose, it boots into Windows.
What would you do to install Zorin OS 15 under these circumstances?
After booting into Windows, can you disable Fast Boot from the control panel settings:
https://www.asus.com/support/FAQ/1008831/
Exiting Windows (reboot); disable Secure Boot
https://www.makeuseof.com/tag/disable-secure-uefi-dual-boot/
And change Boot Mode to "Legacy"
https://support.hp.com/us-en/document/c03653226
Test if USB is recognized and Bootable.
If still not booting, please ensure you have burned the USB drive as a Bootable drive (I recommend Unetbootin).
If your machine came pre-install, I think it was installed in UEFI mode, not legacy. If that is the case, the Zorin live USB should be written for UEFI.
See, for example:
I have made a subtle change to the title of this thread, to add "PC". Just to make it clear PC wont boot anything from USB.
I used the recommended balenaEtcher. Would that not have created a bootable USB drive?
I am using a 64 GB USB drive. Is that too big?
The rufus portable version does nothing. It shows data about the USB drive but no option to select the ISO image or to start doing anything.
Rufus logs say drive has an unknown master boot record.
It's supposed to. But, as a search of the forum can show, BalenaEtcher does not have the best track record. I recommend Unetbootin. It's very easy to use, informative and is available on Linux and on Windows.
Your system may be using UEFI, not MBR (Master Boot Record). Try creating the USB drive as UEFI and ensure in your "BIOS" settings that you are not in Legacy. Then plug in the drive and from the Boot ORder chooser, choose the UEFI mode of the Flash disk as listed on the options.
This time it gave me the option to boot from the sandisk USB and it worked without any errors. I am installing ZORIN 15 Ultimate now.
Actually, Etcher killed 2 of my USB drives 
I cannot understand why it is still listed as a recommended USB writer. I use MintStick in Linux and Rufus in Windows.
I usually use either 8GB or 16GB but there should be no problem using a larger size. It just takes a longer time to format and there will be a lot of unused space.
I've never used portable version, so I have no idea.