I have an old Win XP PC that I wiped the drive on (thoroughly). I downloaded Ubuntu on another PC and copied it onto a DVD. When I try to boot on the old XP PC from the Ubuntu files on the DVD I get an error message, something like the boot failed... duh. I selected the boot device after selecting the F10 boot option when I power up the old PC. Still, the same error message. If you can help, please keep it simple. I am only an intermediate user.
You don't just copy an ISO file or it's contents to a CD/DVD, as it contains a complete file system, with a boot sector, that's why it's called a CD image.
Note: If your computer is not ancient, it should be able to boot from an USB stick, which is often a better alternative to CD/DVD, as it's faster and more flexible, since you can re-format it later.