How do you shutdown a computer on lan from another system on that lan?

start / run / shutdown -i

then just search for comps on the lan, or put in the IP's if you know them off the top off your head.

You can goto this site http://www.suhas-tech.blogspot.com and scroll down there.It is fully explained there.

or you can use special softwares as "Remote Computer Manager" and shutdown remote computers with one click.