Just one other note for putting the case on its side. This isn't necessarily a must do, but if you have something to prop it up at the corners so it's not sitting directly on the ground, you may want to. Frynasyer is correct, as upright cases were actually the second design for home PC's. They were in fact originally designed as horizontal monstrosities. I don't think they cooled as well as an upright with correctly engineered vents, but I suspect the main idea was just to clean up the desk space they took up.

Anyhow, back to the point, your power supply should have a fan on it also if it's not terribly old. If you're technically apt to install a new one, try unplugging the fan (if it has one) first before replacing the whole unit. Fans tend to go hawwire first, mechanically, for maintenance parts before anything else, and they're cheap to replace. As a moving part in your PC that's on contantly when the power is on, they're somewhat expected to be replaced every 3-5 years.

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