Well, I loaded update 2 of Visual Studio 2012 and selected XP compatibility for the resulting project.... and if you run the exe on an XP machine you get a "The procedure entry point InitializeCriticalSection could not be located in the dynamic link library KERNEL32.dll"
Oh dear, so after reinstalling VS2010 onto my machine, up popped this solution from tedwvc
How to get Visual C++ 2012 Update 2 statically linked applications to run on Windows XP
So, if you now re-download ECUFix and install it to your Windows XP machine, all should be well. The slight downside is that you need to ask for another license.