Ok a bit confusing so can we break it down a bit better.
1) if you log off from any user account even an account with admin priviledges, the system does not try to shut down. It only stops the specific user profile from running so that another user profile can access the computer.
2) If you are running the computer admin (different than user accounts) you can not log off or on with that account to switch users as this is the built in admin account.
3) manually shuting down issue...Do you recieve the mesage that "It is now safe to turn off your computer"
if so this usually only shows up in computers that use the older AT format motherboards...you might need to do a BIOS upgrade and re-install windows to correct this issue....
There is another option but it is dependent upon the BIOS abd motherboard format that is being used as to whether it works or not! Read the following from Microsoft..... "It is Now Safe to Turn Off Your Computer" error message when you try to shut down your computer