I installed Ubuntu 12.10 alongside Windows 8 after this guide. Now I can't boot Windows 8 any more. When I choose it in the GRUB screen, it shows me some kind of error, and after pressing enter it says: "A disk reading error ocurred, press ctrl+alt+del to reboot". When I reboot, it's just the same again. Ubuntu is booting normally...
Yeah that link was for win7 and Ubuntu 12.04. There is a problem with win8. Microsoft Windows 8 really created a mess for all Linux distributions.Your system has UEFI activated.which has to be deactivated. Follow this to install Ubuntu on a system which has win8 preinstalled. You will have to follow the steps as mentioned.You will also have to re-install Ubuntu.