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Right-click on the Desktop and choose New | Shortcut from the pop-up menu.
Browse to the file C:\Windows\ System32\Shutdown.exe, click Next, name the shortcut, and click Finish.
Now right-click on the new shortcut and choose Properties.
In the Target box, append the command line switch -l (to log off), -s (to shut down), or -r (to reboot).

If you also add the switch –t xx (where xx is a number of seconds), Shutdown.exe will display a warning and count down the specified number of seconds before activating.
You can specify a comment to be displayed with the warning by adding the switch -c "Your text".
The countdown behavior is particularly useful if the shutdown program is launched through the Scheduled Tasks applet.
For example, a library computer might be scheduled to shut down 10 minutes before closing, with a 60-second warning.
Once the countdown has begun, the only way to stop it is to launch the program again with the -a (for abort) switch on its command line.

C:\WINDOWS\system32\shutdown.exe -s -t 04 -c "Windows XP Shut down: Workstation# 101"

For Restart instead of the -s switch change it to a -r switch as follows:
C:\WINDOWS\system32\shutdown.exe -r -t 03 -c "Windows XP Re-Start: Workstation# 101"

There are also Free S/W for remotely shutting down PC over your Network.
Here are a few:
http://www.goesp.com/downloads/Software/XP-Remote-Shutdown-Tool/
http://www.newfreeware.com/utils/1325/