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: