Ripped from another site:
The new Microsoft keyboards assign actions such as Save or Open to the function keys. To enable the default functionality of a function key without performing the action that Microsoft has assigned to the key, you must press the Function Lock key before you press the function key. Unfortunately, the new keyboards don't contain a programmatic way to set the state of the Function Lock key. Therefore, you can't configure the keyboard to enable the Function Lock key by default; you must press Function Lock every time you start your machine.