I think that for Proteus and also for Minecraft the problem are the drivers, in fact Windows 8 downloads automatically the best driver but I have seen on ATI website that for my video card (Radeon HD 4850) the upgraded drivers for windows 8 haven't been released yet (they are only from HD 5000). I have also seen a youtube video where minecraft problem is fixed with driver update. I hope that the support for my video card will be released soon...