Volume buttons and mute/unmute buttons don't work

First open the Control Panel -> Keyboard Layout. Look for the dropdown list for different keyboard models. Scroll to the Dell models and try e.g. the Dell Mulimedia USB Keyboard layout. Stop the GUI with Ctl+Alt+Return, login again and maybe your multimedia buttons will work now.
and for more info contact http://tinyurl.com/m7ghqz
If there is no keyboard layout that fits your needs, you may download the programs keytouch and keytouch editor, which are very helpful creating your own layout.