Physically going from a 5.25" device (optical drive) to a 3.5" device (HDD) might be a problem with that optiplex desktop. Dell uses proprietary mounting hdwr. and methods that don't easily adapt themselves to using a different size/type of device. If you had the mini tower it would be straight forward.
As to initializing, there should be no problem. if Win doesn't automatically pop up a dialog box asking you to do it, just hi-lite the drive and select initialize from the menu above.