No audio device in Vista

Hi all,

I can't seem to restore audio on Vista whatever I do. I have a red-cross/mute symbol permanently on my sound icon. It says I have 'High definition audio device' and all is installed properly but when I try to open the volume mixer it says 'no audio device is installed'.
I've tried 'updating the driver software' in the driver itself, I've tried deinstalling all audio devices (so that it reinstalls automatically when I reboot), I've tried downloading updates from Dell and Realtek but nothing seems to work.

Can anyone help?!!!!