I'm trying to install Windows 7 (to get rid of Windows 8). I started the installation, cleared the minefield Microsoft has put in to stop me uninstalling 8, but forgot that my Windows 7 disk is an upgrade. So now I'm part-way through the 7 install. Can't go back, can't convince Microsoft that I have a valid key; I tried stopping the Windows 7 install and booting XP first, but I can't either - the unfinished Windows 7 install prevents that. Where's the way out? Update: Having spent some time with a Microsoft operative, I know why I can't install XP; this recent computer doesn't support IDE SATA mode, only AHCI. So the problem is: - straight install - key not valid - XP first before upgrade - XP cannot install due to newer HDD interface.