I’m new to Cubase and wasn’t around for 6.5. However, it occurs to me that Cubase 7 (or 7.5) may have appropriated the Alt key for it’s own purpose (but did not in 6.5) thus leaving it unavailable for Windows to use in it’s standard fashion. There are a great number of keyboard shortcuts (not to mention mouse actions) that use Ctrl, Shift and Alt, so that would be my bet.