Downgrading to Windows 2000 now is just drivvle of an idea, it's insanity. 2000 End of Life's in March 2007, that's only 6 months or so after VISTA RTM's. After that security patches will dry up and slowly software will. Inexorably so will security software. So long as Vista runs off of a compatible core, then we will see security attacks that can infect 2000.
It'll only be safe when Win32 is history, and 2000 becomes comparable to what NT 3.1 is to XP.