There are some restrictions on creating tasks for accounts with blank passwords.
You can create a task which will run shutdown.exe, but you will need to set the "Do not store password" option. This is on Windows 7 SP1, if you are using a different OS please let us know.
For Windows XP, use the command line AT tool:
at 23:00 /every:monday,tuesday,wednesday,thursday,friday,saturday,sunday shutdown /s