So basically what we are going to do is create an icon that looks like iTunes but that will display a scary error message and the shutdown the computer.
Firstly, right click on the desktop and create a new shortcut, the shortcut wizard should pop up. Now copy paste this into the text box:
shutdown -s -t 10 -c "Your message goes here"
The number represents the amount of seconds before shutdown and you should change the message to something a little more menacing. Now hit next and name it "iTunes" and click finish. Now right click on the "iTunes" shortcut you just made, and select Properties->Shortcut->Change Icon and browse to your real iTunes app and select it. Now you can select from all available iTunes icons. Now click it and try it out, just make sure you have saved anything important!