Is it possible to uninstall completely Windows 8 and then install Unbuntu 12.04 LTS (or 12.10) on to the hard disk from a USB flash drive?
Windows 8 is, by far, the worst operating system I have ever come across.
I would make sure the BIOS/MB recognized your Unbuntu USB as a bootable drive first.
Try going into your BIOS at start up with the USB drive connected and booting directly to it.
If that works, it's just a matter of saving your files on an external drive, and installing Unbuntu fresh.
-No need for an official uninstall of windows 8 unless you have a need to make all existing data unrecoverable. Then you would want to "wipe" the entire drive rather than just do an "uninstall" of the OS.