i got the same problem with my vista laptop. processor at 50-90%
on idle.
i manage to do a work around by disabling my network before i
shutdown
apparently, there is something on my laptop that eats processor
power when it detect a network on start up so i disable network
connections on start up and wait a few minutes after start up then
manually enable my network.
or
it could be the vista file indexing thing eating your processor
power.
look for a guide to disable vista's file indexing function