Change the calculator mode from "MathIO" to "LineIO" by pressing the buttons [ SHIFT ] followed by [ MODE ] and then the number [ 2 ] for "LineIO". In "LineIO" Mode the result of a operation is always shown as a decimal number. I know this doesn't match what the manual says, but it works. See Q11 of the URL
At least for me, when i hit [S-D] that changes it from scientific to decimal.
Many friends have a similar problem, when the calculator displays all answers as a fraction. This worked for all of us, so it should work for you, too...