Another post about upgrading to Windows 10, this time on an Oracle VirtualBox. Well, after several failed attempts, the following setting solved it for me: In the settings of the virtual machine, click the “Enable PAE/NX” to activated (find the detailled description with a screenshot here: http://www.eightforums.com/installation-setup/4032-your-pc-needs-repaired-error-0xc0000260-2.html?s=c8602c42ab3c48baaa23fbbb324e5557.
Please be aware though, that for this to work in the BIOS of your host system the virtualisation support has to be activated (e.g. AMD-V, VT-x). After that, the installation was snooth and made not any problems whatsoever!
Find information in English on the 2-click solution here: http://yro.slashdot.org/story/11/09/03/0115241/heises-two-clicks-for-more-privacy-vs-facebook