First create your Windows system recovery DVDs from the HP support assistant and then delete the recovery partition that would be around 20GB
If you want more space for your Ubuntu installation than shrink the windows partition using Windows administrative tools then computer management then disk management
Now you will have enough space to install Ubuntu as UEFI only.
Enable secure UEFI boot in your BIOS and whenthe system is just booting press F9 and choose boot from Live USB as
EFI and install Ubuntu.
Choose "Something else" during the installation and create an
ext4 partition and mount it on
swap at least equal to the size of your RAM and no more then twice your RAM.
Then the system will ask you to create 100MB partition more and told you to marked as boot create that also after finishing installation go to BIOS and enable legacy mode also just when system is booting press F9 it will show you choice of the operating system to boot
osbootmanager is for Windows and Ubuntu choose any one and boot if you will not choose anything windows will boot.
Note: Preinstalled Windows 8.1 is installed as UEFI in a way that even without secure boot UEFI,boot-loader of Windows can boot as legacy also.
If you don’t like this method you can download the
bootnext utility for dual boot after install
However I’ve not downloaded it as of now just registered and waiting I am booting as what I’ve described as I am also using HP-15 notebook