I'd like to understand why Win XP reports something is not a valid Win32 application.

Relevance to W10 - the application is built on W10 using VS2017. I thought I had taken appropriate care to not use APIs that aren't available in Win XP. But when it's installed on Win XP SP3 windows gives me a nastygram saying it's not a valid Win32 application.

So what steps should I follow to determine why this is happening?

I came here 'cos I believe that there are people here who know this stuff.