Assembly language is about as close to the ones and zeros of computer speak that you can get. Here's a sample:
Most people say "Huh?" to that. It is pretty obtuse, but with assembly language you are directly manipulating bits and registers on the processor.
The C:\> prompt you see is from DOS and the command interpreter, COMMAND.COM. When the computer starts, the BIOS gets thing moving, sees what peripherals and components are attached to the system, but can't really do much more. It thus has to look for an operating system to boot from, on either a floppy, a CD-ROM, or the HD. That is why, if you leave a floppy in the drive when you reboot, you will get the "Non-system disk" error, and the boot will stop. That's as far as the BIOS can get you.