Large desktop icons

Hi all,

I'm trying to create an icon file that will scale up to the large-icon
display mode on the Vista desktop but I'm not having much luck.

I can create a .ico file (using Microangelo or similar) and include icons in
256x256 and 128x128 sizes, 32bit ARGB color depth, and PNG compression.
They seem to work just fine inside of a folder in Vista in the sense that I
can right click View->Large Icons and they correctly show the 256px image.

On the desktop however it's a different story - they just stay at the 32px
or 48px size, and don't grow. The only thing that increases is the size of
the box that Vista draws around them. MSWord et al seem to handle this fine
on the desktop.

Any ideas as to what I might be missing (other than a few marbles!)

Thanks for any help that anyone can offer,
-Alex