It?s been awhile but if I remember correctly most AT cases used to use nylon standoffs and the screw use used to keep the motherboard from moving the standoff out of their slots. Once the screw is out see if you can move the motherboard, (up/down left/right) until the standoffs come out of the slots. (I know I?m not explaining this well but you should get the drift if you play with it a little.)
The other way they used to fasten them was the stand offs were inserted into the tray and then the motherboard was placed over the standoff and pushed down so that the top came through the board and expanded to hold it in place. If that?s the case you sweeze the top of the standoff, (the end of a BIC pen works well once you remove the ink cartridge) and gently pull the motherboard off the standoff.