Nintendo, like many manufacturers, employs the use of security screws when they assemble the Wii or any of their other products. They do this mainly to prevent the average-joe from taking it apart. Nintendo has settled upon a screw that uses a special little y shaped screwdriver to remove it. Although it is commonly known as a nintendo screw, it is more correctly referred to as a tri wing (or tri-wing) screw.
Although many types of security screwdrivers are commonly available at local hardware stores, nintendo seems to have found one that no one else has any interest in using. If you search the internet you can find a few alternatives that will work with varying degrees of success. However, the best bet it to get a real tri wing screwdriver and keep it in your toolbox for whenever you feel the urge to hack into your Nintendo Wii.