Why don't .ico files created in Icon Composer work on Windows?

For a couple of months now I've been creating favicons using Icon Composer (which comes with Xcode). It can save .ico files, and I know that sometimes Windows shows (or used to show) larger versions of the icon in some circumstances, so I thought I'd do it that way instead of just using a tiny .png.

Imagine my surprise when a Windows-using coworker tells me that he can't see any of the favicons I've created this way. No Windows browser seems to render them correctly, and only Firefox even attempts to render them at all.

So are .ico files produced by Icon Composer not supposed to be Windows-compatible? If not, is there any quick way to convert them into versions that are? If so, what might I be doing wrong?

UPDATE: Here is an example of a broken .ico file produced by Icon Composer. My coworkers are all on Windows 7, so it's not just a Windows XP issue.

UPDATE 2: This happens with .ico files saved from both Icon Composer 2.1 (Xcode 3) and 2.2 (Xcode 4). In Firefox on Windows, display of these icons is corrupted; the icon is shifted to the right several pixels, and the right side is cut off and wraps around to the left side, as seen here:
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Do you get the same issue if you try Hobiconer to export your icons? Superior to Icon Composer in every way, and free.

The commercial successor of Hobiconer called Icon Slate is $4.99 in the App Store, however Hobiconer is still out there for grabs (hence the link to Softpedia's mirrored file)