My VS2015 code won't run on XP or 2003 - why?

You will find that native code compilations built with with Visual Studio 2015 won't run on any variant of NT5, e.g. Windows XP or Server 2003, despite you having run the VS2015 redistributable on the target machine. This can be very frustrating and confusing.

The issue is that VS2015 by default uses the platform toolset appropriate to NT6 and above, and that will lose you support for NT5. To work around this, you have to modify the platform toolset for your builds. You do that via Project~Properties~Configuration Properties~General~Platform Toolset. As they say, (picture==(word*1000)), so see below:

The default setting is Visual Studio 2015 (v140). If you select Visual Studio 2015 - Windows XP (v140_xp) instead, then you will regain the ability for your code to run on NT5. Of course you may question the wisdom of running on platforms that MS itself no longer supports, but the poor grafters in corporate environments often do not get this choice.