Just Go to Sleep Already!

No, this post is not about my children, its about my computer.
Anybody out there that is computer savvy know why my computer refuses to go on standby or hibernate? I have windows XP and I go to "power options" on the control panel and pick the "energy star" option. It is supposed to go on standby after 20 minutes and hibernate after 1 hour. "Enable Hibernation" is clicked and I have enough disk space. Even when I go to the start button, click "turn of computer" and click "standby" it does not work. It goes to standby for about 1 second before it just comes back on again. I hate listening to it running all day and I hate knowing I'm wasting all that power. Any ideas?