Taos, a member here and on Exetools, has posted the following warning about an update to Windows Vista 64 and Unsigned Drivers:

Do not install this update, reason? :
Be careful of the kb932596 "update" it stops the "bcdedit -set load options DDISABLE_INTEGRITY_CHECKS" option working, that a lot of vista x64 users were using to load unsigned drivers, and the associated MS KB article doesn't see fit to mention the fact that this is probably the only thing this ""update" does. KB932596 definitely breaks unsigned drivers.
If you use google, you will see a lot problems with this update.

Fortunately, he also posted a "solution":

Enter into Programs & characteristics (programas y caracteristicas) under control panel (panel de control) add/remove applications.

Then see installed updates (ver actualizaciones instaladas). Click on kb932596 and then uninstall. This will remove this bug. ;-) "

Thanks taos!