Leave it on or shut it down?

I turn my desktop computer (running XP) off every day when I'm not using it.  It is a pain to wait for startup and shutdown times, but I'm concerned for the environment and all that. I think most people leave it running 24 hours.  I can understand why.  Sometimes it take over 10 minutes to really get all services loaded.  I've tried hibernate, but the Dell 9150 I use comes back from Hibernate with the fan running at full speed and it's very noisy.  Windows seems to be geared for 24 operation-- with automated processes kicking off at different times of the day.  It's like the fridge-- but I'm not sure how it compares in terms of energy consumption. I've actually set my bios to kick on at 4am so the computer has finished start-up before I get up there ten minutes later.  It does all my site backups at a specific time. I'm trying to find more energy efficient uses of the pc.  If anyone has any ideas, I'd love to know.