Folks, I feel like a lot of variants of this question have been asked but that the specific cases are pretty divergent, and nothing I've found on these threads has solved my issues -- so here goes.
I've got a new computer -- an HP Pavillion with Windows 8 preloaded, made in late 2012, with an AMD Trinity A10-5700. I've set up quite a few dual-boot computers with XP/Ubuntu and Win7/Ubuntu -- you always need the Windows for crap people send you from work! -- and that's easy enough but I get that "Fast Boot" and UEFI create all kinds of fascinating issues for Windows 8, though I'm not BIOS-savvy enough to really get those issues.
So: I made an Unbuntu installation CD (under Ubuntu, from an older Win7/Ubuntu machine). Downloaded 64-bit 13.04 (I've tried all this stuff with the 12.04 LTS too, I've gone to 13 because I know there's a UEFI support issue). If I disable "Fast boot" then I can (UEFI) boot up on the Ubuntu Installation disk and it takes me straight to the menu screen (the "Try Ubuntu" "Istall Ubuntu" "Test disk" jazz.) Cool beans. But no matter what I pick -- nothing happens. The computer hangs, forever.
Disabling "Secure Boot" mnakes no difference (except that the screen flashes "Secure Boot not enabled" before going to that menu).
If I _enable_ Legacy booting, then I can boot up on the install CD, I can try ubuntu and mess around, and I can go through the motions to install, but I haven't completed that process because it's my understanding that I want to install the bootloader under UEFI so it plays well with the Windows partition.
So, why can I only start up in Legacy mode?