Not sure I agree entirely here - Microsoft is definitely a far better bet than Apple for this kind of stuff, but it's still not a perfect bet. As Microsoft has shown with their Win8/phone platform, they are trying to transition away from their old platforms (although the market currently won't let them). It's almost a guarantee that eventually old Office versions will fail to run and after that running into data incompatibilities with old files becomes par. With the call-home DRM in post Microsoft platforms, there is also no guarantee you'll even be able to run an old Microsoft version either.
A better option is to stick to open standards. If your presentation is done in something like HTML, there is a 100% chance you will be able to view it in even 50 years as it is standardized and there are many implementations available for viewing it.
Of course, if your presentation depends on some javascript calculations or a remotely hosted jquery, those probably won't be working in 50 years time either..
Saying Microsoft isn't perfect at backwards compatibility is nit picking to say the least! They are far far ahead of any competition in terms of backwards compatibility. It's practically measured in decades whereas Apple can't manage 5 years.
A better option is to stick to open standards. If your presentation is done in something like HTML, there is a 100% chance you will be able to view it in even 50 years as it is standardized and there are many implementations available for viewing it.
Of course, if your presentation depends on some javascript calculations or a remotely hosted jquery, those probably won't be working in 50 years time either..