Nope. nothing to do with drivers, still does the same thing even on a bare bones install of vista32 sp1.
Also does the same thing on my laptop with pre-sp1 vista.
Here's another oddity, even if you set the slider to min and even mute it as well then close the mixer, then trigger a uac event like going to device manager, the uac alert sound will still play at full volume. But if you open the mixer and leave it open, the uac alert is now muted or at the volume level you set the slider to [if you didn't click 'mute']
Close the mixer ans uac alert is back to full volume!!

This is also the same on the laptop.