Are you trying to burn the boot repair iso you downloaded to a CD? If so, you need to select the option to "burn as an image" and not simply copy it to the CD. Same if you are using a DVD with an Ubuntu iso, it's too large to fit on a CD. To put it on a flash drive, you cannot copy it directly to the flash drive but need to use software designed specifically for this purpose. I don't know what you are using now, what operating system or some Live CD. Some software you can use to create a bootable CD include unetbootin or pendrivelinux, although the latter only works from windows.