Hi there,
Since your PC is new and has an Intel 8 series chipset the motherboard firmware will be based off of the much newer UEFI firmware standard rather than the (comparatively) ancient BIOS standard. Some motherboard vendors continue to list the motherboard firmware on their website as BIOS even though it is clearly UEFI based.
UEFI handles booting much differently than BIOS, but for compatibility reasons some motherboard vendors have included BIOS compatibility with UEFI firmware. This enhances the ability to boot off of old hard drives, old operating systems, and optical images that are not setup for the newer UEFI boot chain, but it has been known to cause issues when the installer encounters a mixture of the two. All installation media for Windows 7 and beyond (obviously including Windows 8.1) include everything necessary to boot through UEFI. If there is absolutely nothing else on the computer (which should be the case if you built it from scratch) the best solution is to disable legacy boot compatibility in the firmware settings. You will have to consult your motherboard manual on how to do this but in general you want to do the following:
1. Disable legacy boot compatibility entirely. You won't need it unless you're installing something that does not support UEFI such as some older Linux distributions, or various BSDs.
You can tell if legacy boot is enabled if you see various disk drives themselves as boot targets. UEFI scans the partition tables on the devices and enumerates individual volumes that identify themselves as EFI bootable partitions. So, rather than booting from "SOME LG OPTICAL DRIVE", you'll want to boot from something like "Windows Boot Manager". It's a much nicer system.
2. Disable legacy boot ROMs if present, or at least prioritize UEFI ROMs. These are the ROMs that control the storage controllers and run in order to put the storage controller into a state from which it may read the bootable media. Some platforms that bridged the gap between BIOS and UEFI have both, with one being prioritized. This is common on Intel 6 and 7 series motherboards. Add-in cards such as dedicated RAID controllers may have both as well, and the firmware may retain compatibility for these older ROMs as manufacturers make the shift to UEFI-only. Disabling them generally won't fix boot errors, but your PC will boot much quicker with all legacy components disabled (BIOS compatibility components collectively form a firmware subsystem named Compatibility Support Module). If you can disable CSM, and enable Secure Boot you will be have a very fast booting, very secure PC.
3. Make sure that the optical disk is attached to the Intel PCH storage controller rather than an add-in controller. I don't believe that yours has one, so you may be able to disregard this along with #2 above for the most part.
I hope that this helps.