Volume Buttons on Keyboard not working

I had a Wireless Keyboard and Mouse hooked up to my old dell windows xp
Dimension and the plugs on the Keyboard and mouse wireless transmitter were
both PS2 type not USB.

Now I have a Dell Vostro with Vista and of course it doesn't have any PS2
ports so I had this neat little adaptor which converts the PS2 keyboard &
mouse plug into 1 USB plug.

All is working great except for the volume up & down and mute buttons have
no effect in vista at all whereas they did in XP. come to think of it no
multimedia buttons (like play stop etc.) work on the keyboard now with
vista. I even went to the manufacturers website and downloaded a driver for
the wireless keyboard and mouse but still nothing.

Is there something extra I might have to enable in vista to get these
buttons working would you know or some kind of software patch?

Has anyone seen a PCI to PS2 Keyboard & Mouse plug in card (if such an item
exists?) just in case this solves the problem?

Thanks,

Andy.