Why does my BIOS Boot list show
I have disabled it because it slows down the startup time. Why is it still there? How can I boot from it? I am confused.
Its there because some corporate networks support network booting, which means it grabs everything it needs over the network.
See Wiki's writeup: http://en.wikipedia.org/wiki/Network_booting
You can use your "Ethernet Boot option" to boot linux based systems such as Clonezilla, PartClone, Ubuntu, and a lot of systems and aplications based on Linux PXE(Preboot Execution Environment) without needing to use a Hard Disk on your computer, all the kernel and dependencies will boot up to your RAM from the Ethernet Server, as explanined above:
You can disable this option on your BIOS if you dont want to use Ethernet boot based systems, actually it would be disabled from factory configurations on most of manufactures.