Force Ubuntu To Install Legacy?
Solution 1:
First, verify that Windows is booting in BIOS/CSM/legacy mode. Windows 8.1 booting in that mode is certainly plausible on a self-built computer, but you should be 100% certain of what you're dealing with.
Second, the simplest way to get Ubuntu to boot in BIOS/CSM/legacy mode is to remove the EFI boot loaders from the Ubuntu boot medium. If you're booting from a USB flash drive, deleting the EFI directory tree on the flash drive should do the trick. There's also a Mac-specific version which, ironically, is BIOS-only, so you could download and use that.