I have reverse engineered this patch and its effects on keyboard layouts a bit. The patch works by shipping a new version of win32k for XP and Server 2003 that pay attention to a registry key. When this registry key is added, it restricts loading of keyboard layouts to the System32 folder (already done in Vista and later). This prevents further exploits on the keyboard layout loading code. This is the first part shipped in KB2676562.
The second part is a patch (KB2686509) that adds this registry key. Before this registry key is added, a DLL called kblchecker.dll is loaded that is shipped inside the patch. This DLL is supposed to enumerate all the keyboard layouts on the system and make sure they are all in the system32 folder because any other keyboard layout DLL is going to be disabled by this update. What I found out by black box testing this patch is that any registry key value (not subkeys or any value inside a subkey) in the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Keyboard Layout key regardless of name is going to make this check fail with no FaultyKeyboards.log being created, which looks like a bug. The reason MS is not fixing this bug is probably because all it does is makes the installation of this patch fail.