It's already in Windows (well certainly WindowsXP). Look in the Control Panel for Scheduled Tasks.
If you want to be able to shutdown at a specific time you can use the "shutdown" command.
Here is the shutdown command syntax:
Usage: shutdown [-i | -l | -s | -r | -a] [-f] [-m \\computername] [-t xx] [-c "comment"] [-d up:xx:yy]
No args Display this message (same as -?)
-i Display GUI interface, must be the first option
-l Log off (cannot be used with -m option)
-s Shutdown the computer
-r Shutdown and restart the computer
-a Abort a system shutdown
-m \\computername Remote computer to shutdown/restart/abort
-t xx Set timeout for shutdown to xx seconds
-c "comment" Shutdown comment (maximum of 127 characters)
-f Forces running applications to close without warning
-d [p]:xx:yy The reason code for the shutdown
u is the user code
p is a planned shutdown code
xx is the major reason code (positive integer less than 256)
yy is the minor reason code (positive integer less than 65536)
