For what it's worth

My daughter's HP Pavilion G6 Windows 7 laptop suddenly started sounding like a washing machine on its spin cycle. The function keys wouldn't mute or cut volume. We ended up shutting the laptop in another room until it had finished its update and could be forced to switch off.

This was a howl-round, laptop style: the microphone was picking up the internal noise of the laptop, which was amplified and fed to the speakers, which the microphone then picked up and amplified ... and so on.

Not sure how we set up the situation, but the solution was to disable the Stereo Mix on the Recording section of the Sounds section of the Control Panel. "Stereo Mix" is a channel that takes the output that would go to the speakers and treats it as an Input. It is potentially useful in that it should allow you to record from streaming audio (eg from BBC iPlayer radio programmes).

But if you ALSO have the internal microphone configured with "Listen to this device" set on, then you get the howlround.

How amazing that Windows 7 sets up such an obvious pitfall without any warnings when the settings are configured to make this happen!