To enable logging to the security event log of code integrity related events type this from an elevated CMD:
auditpol /set /subcategory:"system integrity" /failure:enable

I believe this will allow you to determine when a driver fails to load on x64 SKU's of Vista for whatever reason (cough - not signed - cough).  May prove interesting some day. :)  I need to reinstall QuickTime + iTunes and see what happens - I think I've read about it using some driver for DRM related tasks or some such . . . want to see what happens on Vista.