Can't boot Windows 8, can't boot from USB installer

I'm having all sorts of problems with my PC. It's custom-built so I didn't expect to have UEFI/Secure Boot compatibility issues but I think that's what's happening.

I have a Windows 8 Pro installation on C:. I tried installing Ubuntu 14, which did two things:

• choosing Ubuntu in grub2 takes me to a black screen
• choosing Windows in grub2 takes me to an error screen:

0xc000000e: The boot selection failed because a required device is inaccessible.

I then installed Ubuntu 12 over the Ubuntu 14 installation to se if that would resolve my gfx issues.

Now booting up takes me directly to grub-rescue.

I made a Windows 8 setup USB drive by using the Windows 7 USB Download Tool and the windows 8 ISO, which has worked for me in the past. But trying to boot from that device just immediately takes me to grub-rescue again.

I have tried:

• using oscdimg.exe to make the ISO UEFI-bootable
• several permutations of options in my BIOS
• burning the ISO with Rufus as GPT for UEFI

With Rufus, the USB drive at least takes me to a Windows boot error screen again:

Your PC needs to be repaired. An unexpected error has occurred (0xc0000225).

I thought the Windows 8 installation ISO was capable of startup repair, but it just takes me here.

The following are my potentially relevant available BIOS options (GX-Z87X-UD3H):

• Fast Boot: Disabled
• Windows 8 Features: (Other OS, Windows 8, Windows 8 WHQL)
• • If Windows 8 or Windows 8 WHQL:
• • • CSM Support: (Always, Never)
• Boot Mode Selection: (UEFI and Legacy, Legacy Only, UEFI Only)
• Storage Boot Option Control: (Disabled, UEFI Only, Legacy Only, Legacy First, UEFI First)
• Secure Boot state: Disabled
• Secure Boot: (Disabled, Enabled)

I haven't found a permutation of these options that lets me boot the USB drive.

I can also set a custom policy to "query user" for removable media, but that seems to have no effect.