.NET 4.0 runs on XP and it is still very easy to create a .NET 4.0 application on a more modern machine. A well tested .NET app will deploy and run on Windows XP with few surprises. You cannot ask for better tooling. So, I would not say that creating new software for XP is really all that hard.
If you want to be much cooler but put in more work, check this out!