Is there any way to write a WinRt (Metro) app that will also work on Windows 7 and Vista?

Question

We can’t just leave our customers that are not able to upgrade to windows 8 for a long time in the larch. However there is demand for a “tablet”/”touch” versio

Answer 1

The only way I can think of is to implement your application in HTML5/CSS3/JS, and avoid using WinRT APIs inasmuch as possible - this may be feasible depending on what, exactly, your app needs to do (e.g. portable 2D graphics is easy with HTML5 canvas).

Then, for Win8, you'll package this as Metro web app. For Win7 and below, you write a simple app that embeds your browser of choice (not IE9, since it doesn't work on XP - so Firefox or Chrome) with all chrome hidden, and loads your HTML5 app inside that embedded browser.