Active noise cancellation for a computer?

Hi all,

I was just pondering, has anyone tried to silence their computer thru active noise cancellation?

I've done all the normal tricks, watercooling, hard drives on elastic, low rpm fans, but I still hear my computer as I use open headphones...

I was wondering if there is a way I could somehow silence the computer? I was thinking mount a mic in the computer case (I have a Sanken cos 11 lavalear mic), and them maybe supply the inverse signal to the sound going to the headphones? Or even a speaker in the PC case?

Also, I really need to try recording some fan noises, but would I be right in assuming a fan spinning at 600RPM has a fundemental frequency of 10Hz? (600/60=10)