Reasons of Corruption are Obscure until now..
You may have to Rebuild the Icon Cache in Windows 7
If one or more of your icons are not
displaying correctly, or that your
icon cache is corrupted, then you
might consider rebuilding the icon
cache to reset and reload the icon
images into the icon cache.
This will not help with any of the following icon issues:
1-A shortcut's icon that is not displayed properly due to the source
of the shortcut being moved or deleted. You may need to recreate a
new shortcut directly from the source
(ex: program's exe) file to replace it
2- All icons for a specific file extension display the wrong icon. This
may be do to setting the wrong
association for what program to open
the file extension instead. You can
restore the default file extension's
associations to fix this icon instead.
If you have check that the source icon is not corrupted use this solution:
Windows 7 - Icon Cache - Rebuild from the SevenForum tutorials