If you recently added an icon to your WPF project (any .NET version, including .NET 3.5, and .NET 4.0) and it has support for an alpha channel (often referred to as the Vista icon format), stop. Why? Your WPF application won’t run on the latest service pack of XP as it’s not capable of decoding the format unfortunately. It’s a very frustrating error and a stupid "feature that Microsoft overlooked. I’ve hit this a few times unfortunately.
To fix, remove all of the alpha channel images from the ICO file and recompile.