I meant as a developer platform. Yes it's true that, depending on the way it's implemented, it's common that software can't be installed on the exact versions the vendor tested it on or 'approved' it for. What I meant was that Microsoft bends over backwards to provide compatibility, although I guess I should qualify that by saying 'at the lower levels', i.e. on C/C++ code written not too far away from the OS dll's. Silverlight of course is a higher-level counter-example.