When installig XUbuntu 15.04 *64, I choose the installation alongside the existing Operating System (Windows 10). It does not rcognize any O.S. and asks me to delete all and install it. What can I do?
There are virtual OS solutions where you run W10 as host and within it you then run guest OSs. That's how I run XP on my main W10 PC. If it goes wrong you still have your main Windows 10 system up and running. I use VMWare workstation which is free but there are alternatives available. You can install additional OS using an ISO when running VMWare workstation. After booting up W10 you open VMWare Workstation and launch the OS that you want to use.
I have VMWare Workstation running on a W10 system. Dual booting is not needed and would introduce the possibility of not being able to boot into Windows if something corrupts. Maybe you think Ubuntu is better than Windows so you could use that as the host system instead and then install W10 as a virtual OS accessible once you boot into Ubuntu. I'm sure a few enthusiasts have done it that way round. For me the DIY OSs are far behind Microsoft Windows. I have no separate partition for my virtual OSs and my computers boot directly into Windows 10.