Okay, by activating Secure Boot, I now have access to GRUB! Success!
There is still an issue though... Lubuntu doesn't boot, except on the pendrive, and all of the options to start Windows 8 return an error and take me back to GRUB.
Well, that wasn't fully accurate. Lubuntu DOES boot, but it doesn't finish booting. Partway through the boot process the screen freezes for an indeterminate amount of time(probably eternity until the computer is shut off). I have a lot of extra options in GRUB as well, most of them HP garbage I can easily ignore.
So for the most part, it's working, but Lubuntu doesn't want to boot. Any ideas or diagnoses I can run?