One for the Googlers:
My Canon Powershot A630 also records videos. Unfortunately the microphone catches the sound of the internal mechanism (I think it’s the screen) and the video it produces has an annoying ringing noise.
I extracted the audio from the video using VirtualDub, and opened it with Audacity 1.3.4 beta. Like the Wiki page instructed, I used the Nyquist prompt and the command
(notch2 s 4290 50) (yes the brackets are necessary), and this removed the noise flawlessly!
All that needs to be done is now to reinsert the audio into the video file. I can combine this with re-encoding the video with a more size- (and web-) friendly codec.