Shutting my Computer Up

Thus far, the only thing that's annoyed me with my new computer is the damn fan noise it makes. It gets loud. Very loud. So loud in fact, that I can hear it on the floor below me and at the front door (though thankfully not outside it).

So my father had the idea to take parts of the case off to see if that would help the ventelation, and so far it has. We've taken the left side off (if the system is facing you), and while the processor still generates a lot of heat, it takes a lot longer to build up and kick the fan into overdrive. I can actually play Doom 3 for hours on end and the fan remains at its normal, not annoyingly loud volume.

The only exception to this so far is Half-Life 2. My father's playing this, and shortly after booting the game up, bam, the processor's fan goes nuts. Now, the only reason I can think of for this is because of the physics in the game, which is rather CPU intensive from what I've read. Basically, Valve's Pretentious Physics Engine (TM) gets my CPU all steamed up.