Ubuntu 15.04 seems to install better with windows 8.1. So, I was able to install it once and it came up with Ubuntu. Then, I also checked if Windows worked as well. However, it immediately went into windows without the choice for the OS part. I reinstalled Ubuntu, but this time it gave me the error message:
Executing 'grub-install/dev/sda' failed. This is a fatal error.
If you installed Ubuntu first and than Windows, Windows has overwritten grub bootloader, so it will go directly to Windows. When you are trying install Ubuntu and you get error grub-install/dev/sda it's probably caused because issues like EFI vs. Legacy installation. Try to use Ubuntu Live CD and install on Live CD https://help.ubuntu.com/community/Boot-Repair and try to repair boot loader, that is if you have not corrupt partitions yet.
It is amazing. I may not clearly have stated that Windows 8.1 was there first. I followed the boot-repair procedure suggested to me in the answer and it seem to work. I did boot both in Ubunntu and in Windows. Now, I just turned the machine on again and it comes up in Windows again without a choice for the OS.
In the process, the message it gave to me was: http://paste.ubuntu.com/118643335
I was sure that the problem was fixed but it is not the case.
