Well ok. I only have two suggestions.
1. Update everything. literally, everything.
2. Shutdown through CMD. The command is: shutdown -s -t 0
It will shutdown in 0 seconds.
I have a Dell Inspiron with Vista and everytime i try to shutdown or go into hibernation the system reboots. shutdown brings me back to the signon screen and hibernation brings me to the desktop. I usually keep aim running and never exit before i shutdown or hibernate i'm not sure if that is causing the issue.
I have also had the same issue of hibernation disappearing from my start menu after doing a disk cleanup but fixed that...is this part of that same issue maybe?