I think Cubase 5 works fine in Windows 8, but don’t take my word for it 
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The desktop is not hard to get to in Windows 8.
The easiest way to describe metro is it’s a fullscreen startmenu, where the desktop is accessible through one of the buttons.
I’m not sure you can upgrade Vista to 8 without a reformat, but even if you could I would advise you to format your drive and install windows 8 from scratch, seeing as your vista install isn’t really stable anymore. An automatic upgrade would try to transfer as many settings and files as possible so there is a possibility it won’t even solve your problem.