From the Visual Studio Developer's Blog you can:
To target Windows XP:
- In Solution Explorer, open the shortcut menu for your project, and then choose Properties.
- In the Property Pages dialog box for the project, under Configuration Properties, General, set the Platform Toolset property to Visual Studio 2012 – Windows XP (v110_xp).
I now understand what has gone on and will update the latest build of ECUFix.