Instantaneous change from checkbox

I would like to make a script that instantaneously changes a value or image on the same page given a user form check. One example is Alienware's computer customization page:

http://www.alienware.com/Configurator_Pages/mj-12_3200.aspx?SysCode=PC-MJ12-3200&SubCode=SKU-EXTREME

Just by box checking specs, it changes the Total price at the bottom instantly. Does anyone know how to do it? Thanks!

http://www.codingforums.com/showthread.php?t=34322

thanks so much, really appreciate that!