Yes, TotalBeginner, forgive me but I fear you are wrong. In fact the drag from a stationary prop is much greater than from a windmilling prop.
Just to back up many of the other responses to this misunderstanding with an illustration:
It takes a lot of force and energy to turn over a non-running engine. Have you ever tried to push start a car? In fact it takes a starter motor about 60 amps at 12v to turn over a decent sized engine - that's a welding current. If a failed aircraft engine is windmilling, the power to keep turning over that engine is coming directly out of your glide performance. A seized or feathered prop will provide less drag.
If the quote above was correct, when helicopters have engine failures, they would put the rotor brake on! Instead they allow the rotor to autorotate to gain minimum rate of descent. Same deal - think about it!