This has been discussed many times already:
@NeilBlanchard:The only thing that can work along these lines is a headphone type setup (and the Bose units used to cost a lot less, BTW -- like $70-80 IIRC). Because in order the cancel a noise, the "antidote" MUST be exactly 180 degrees out of phase, and since the sources of the various noises in a computer case -- are in various places, there is no practical way that a single device could cancel all the noise!
And as you move around, your ears change the distance from those noise sources -- and from the cancelation device, so again, there is no practical way that a cancelation device can work EXCEPT if it is attached to your head!
Me and my father think fundamental is the anti-phase of a single tone sorce.
Noise from a computer is not a pure tone, it is broadband.
Like pollution, it is a lot easier to stop it being produced in the first place than to clean it up once it is in the environment.