I know you're loath to upgrade, but surely you're not still running a CPU that doesn't have an idle state. Modern CPUs (where "modern" probably means "engineered in the last decade") consume much less power when they're not doing anything than when they're running full bore. That's why fans kick on: to dissipate the additional heat generated by the additional power consumption.

Personally, I'm fine with trading that off with the computer doing something, but don't fool yourself into thinking you're getting something for nothing.