I never had to worry about this with my oldest child, but since now I have a computer sitting next to a TV that is freely accessable to all little fingers, my youngest has discovered that she gets a "funny" reaction when she pushes the power button and DVD eject button (both from the computer and from myself ). To help protect against this, I found a little program called Toddler Keys. Once set up it will lock out the power button (XP only), eject button, and keyboard/mouse. The only issues is that it also locks out software eject commands as well, which a simple bat file to close Toddler Keys then open the DVD tray gets around. She is no longer so interested in those two buttons as they don't get the same reaction like they used to...
The reset button though...that's another story , and one with the conclusion of opening up the computer and pulling the connection from the MB . She teaches me so much .