To avoid restarting server during working hours you easily schedule restart to be performed at night. Obviously the same can be applied to a workstation rather than server.

Create a text file restart.txt with text:
shutdown /r /c "Your message displayed before restart"
If you want to shut down instead of restart, use this text instead:
shutdown /s /c "Your message displayed before shut down"
Rename the file to restart.bat

a. For Windows Server 2003 and Windows XP

Start > Programs > Accessories > System Tools > Scheduled Tasks
Add a new Scheduled task
In Task Tab > Browse > select your restart.bat file
In Schedule Tab > Schedule Task > select Once and set the restart date and time

b. For Windows Server 2008 and Windows 7

Start > All Programs > Accessories > System Tools > Task Scheduler > Task Scheduler Library
Create Basic Task
Name: Restart
Trigger: One Time, select Date and Time
Action: Start a program - select your restart.bat file
Finish

When restart is initiated and message is displayed you have 30 seconds to cancel the restart:
Start > Run >cmd
shutdown -a

Windows Server 2003
Windows Server 2008
Windows XP
Windows 7