A new software tool has been released by Linchpin Labs that allows the loading of unsigned and legacy drivers on Windows XP, 2003, and most importantly Vista.
One of the system management and control methods that Microsoft implemented with Windows Vista is requiring system drivers to be digitally signed before they will load properly within the system. If a user or administrator wishes to load an unsigned or legacy driver, they will either need to reboot into a limited functionality mode or just do without the functions that the driver would have provided.
As others have pointed out, this step will do nothing to prevent malware authors from being able to load their drivers into the system. Either they will exploit the lax jurisdiction and corporate oversight of various countries to establish a corporate shell and gain legitimate digitally signed driver certification, or they will just exploit weaknesses in already-signed drivers.