I downloaded Ubuntu 14.04 from an official web page and installed it as instructed by many different sites on a pre-installed Win 8.1 VAIO laptop.
I was able to boot to Ubuntu and play around, even without the installation usb plugged in, but once I switched to Windows, it took over the boot, and from then on I had no way to boot to Ubuntu.
I looked at Windows' system properties and saw that under "default operating system" there was only Windows.
Therefore, I tried to reinstall. The partition I had allocated was still there, but once again, after a seemingly smooth installation process, and being able to load the OS and do actual stuff like installing packages and so on, loading up Windows was the end of the road for Ubuntu.
In both cases I made sure I was installing Ubuntu on the main hard drive and not some other removable media, and also I made sure I didn't choose the "try Ubuntu" option.
What could I have done incorrectly and how can I fix this?
Thanks a lot, and I apologize if this is a duplicate - I couldn't find any reference to this type of problem.