I have an application which I have made a 256 x 256 Windows Vista icon for.
I was wondering how I would be able to use a 256x256 PNG file in the ico file used a
Have a look at the Windows icon functions that are available. There's also an overview that mentions querying for different icon sizes. There's a Dream.In.Code forum thread for using the APIs in C# as well as a Pinvoke.net reference.