It seems very unlikely that VB6 apps (which are a significant proportion of all applications in use today) will not run under future MS operating systems. Remember you can still run most DOS applications under XP. The question I think is whether your users will want to or not. And I would guess that compatability, as it did with DOS programs, would depend on coding the original app 'according to doctrine', e.g. not doing anything 'smart', like using undocumented OS features, addressing h/w directly, etc.