What do you do

If you need to compleatly upgrade a users computer? If you want to upgrade a system from say a 1 gighz computer to a 2.0 gighz system. New mainboard, cdrw, network card, modem.. ect and you get a stop screen saying basicly you can not do this. I have found this on three systems and can not migrate from the old hard drive to the new. I thought that there was a tool to move data from one drive to a new one.