"Young lady, in this house we obey the laws of thermodynamics!" --Homer Simpson, after Lisa builds a perpetual motion machine
Physics doesn't work that way. You could probably extract some energy from the waste heat (by exploiting the thermal difference between the CPU and the ambient air), but overall, you can't get more energy out of something than you put in. Whatever you use to turn the heat into power will generate even *more* waste heat, heating up the environment around the computer until there's no longer a temperature differential (or forcing you to spend an equivalent amount of energy cooling the whole system).