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As the title says, I need to install ubuntu in dual boot with windows 8.1 on my home PC.

I knew that with UEFI partition table was written in GPT, but I found that the partition table on my hard drive is written in MBR, composed by two partitions:

1)BOOT partition, formatted in NTFS has size 1.5 GB and flag boot 2)OS partition, formatted in NTFS covers the rest of the HD.

In UEFI menu(canc at the beginning) Secure Boot and Fast Boot are disabled. I found on the internet that the partition table is MBR if UEFI is setted in Legacy mode, but I can't find this option.

Furthermore, Ubuntu live doesn't recognize any other OS installed on my HD(I booted ubuntu live on a USB stick using UEFI option in the boot menu and it went fine). How should I proceed? Should I convert all the partition table to GPT(I read on the internet it could be done without any loss of data) and then install Gummiboot or should I just install Ubuntu in the old legacy mode and let grub do the rest?Thanks for your help.

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Your easiest path at this point is to do a BIOS/CSM/legacy-mode install of Ubuntu. You can probably do this by selecting a boot option for your installation medium that does not include the word "EFI" or "UEFI." The result will be just as if you were using an older BIOS-only computer. The biggest drawback to this will be slightly longer boot times than you'd have if you were to boot both OSes in EFI mode.

If you want to do an EFI-mode install of Ubuntu, you'll pretty much have to either re-install Windows or convert it to boot in EFI mode. (There are ways to have two OSes installed in two different boot modes, but they tend to be pretty awkward to manage.)

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It is you to decide if you want boot to be based on UEFI or on BIOS=CSM=legacy.

It seems Windows 8.1 is installed in BIOS/MBR mode. You can install Ubuntu also in BIOS/MBR mode.

To convert Windows 8.1 BIOS/MBR installation to UEFI/GPT you need:

Do 1), 2) and 3) with Ubuntu Live CD/DVD/USB

1) convert existing disk to GPT

2) delete BOOT (1,5GB) partition. (assuming it is the first partition on disk)

3) create in the free space (1,5GB) three new partitions:

a) 300 MB EFI System partition

b) 128 MB Microsoft Reserved partition

c) rest to 1,5GB - Windows Recovery partition (would be about 1GB)

4) After this you boot Windows 8.1 installation DVD/USB in UEFI mode, go to command prompt and type:

bcdboot c:\windows - where c: is drive where Windows is installed on HDD, eventually change c: to drive letter of actual mapping.

Now you should be able to boot Windows 8.1 in UEFI mode.

5) Shrink existing Windows partition and on freed space you can install Ubuntu 14.04 in UEFI mode.

Note:

If you boot installation CD/DVD/USB with Linux or Windows using UEFI booting - destination HDD is assumed in GPT style by default

OR

you boot installation media using Legacy/CSM/BIOS emulation then destination HDD is assumed in MBR format by default.

Firmware should allow switching boot mode to UEFI or CSM/legacy/BIOS or both(UEFI+CSM).

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