The Palm Treo 800w can be hard reset by following this procedure:
1) Remove the battery as described in the diagram above.
2) With the battery removed, press and hold the End/Power button (seen left) and reinsert the battery. When the screen displays the Palm logo, release the End/Power button. After a few moments you'll be presented with a screen which asks Erase All Data? Press Up on the four-way navigation pad to confirm. The hard reset will complete.