Comment 8 for bug 811485

Allen, Windows ties its boot support to the partition table type, so if you have an MBR partition table, then Windows is booting in BIOS mode, not in EFI mode. It's entirely possible that the Ubuntu installer started up in EFI mode despite this, though, which could cause problems. The best (or at least simplest, if possible) solution is to find a way to boot the Ubuntu installer in BIOS mode. This might be possible by using firmware options to force a BIOS boot from the optical disc or by using UNetBootin or some similar tool to create a BIOS-bootable (but NOT EFI-bootable) medium.

An alternative solution is to convert the MBR setup to GPT, convert Windows to boot in EFI mode (in-place or by re-installing), and then install Ubuntu in EFI mode. This is likely to be a pain to do, though, particularly if you're not an expert on EFI-mode booting and installation, so I don't recommend it unless you're desperate.

As far as Ubuntu development goes, this does raise an issue: Even though the installer boots in a given mode (BIOS or EFI) does NOT necessarily mean that existing OSes are installed in that same mode. It could be very tricky for the installer to detect this and install the correct boot loader, even if that means installing the boot loader for the mode that's NOT being used.