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Peak vs. Continuous Power
When you read the current (or power) rating of a device such as a hard disk, you are usually seeing the manufacturer's claim of how much the device uses during normal, continuous operation. The actual peak draw of the device is at its maximum, however, at startup--not while it is running. The +12 voltage provided by the power supply is used primarily to drive disk drive motors. Because of their design, these motors can take up to double their steady-state current when they are spinning up from rest. If you have 3 or 4 hard disks in your system and all of them start up at the same time, this can be a tremendous demand on your power supply's ability to provide +12 V power.
Fortunately, most power supply companies take this into account and build into the power supply the ability to exceed its normal output for a short period of time during startup. You will usually see this specified as a "peak" rating, often only for the +12 V line, where this is a particular problem. (It is much less of an issue for the +3.3 and +5 voltages, and therefore many power supplies do not specify a peak rating for these voltages.)
Despite this extra capacity, it is still a good idea to not load up your system to the very limit of your power supply's stated power capacity. It is also wise, if possible to employ features that delay the startup of some disk drive motors when the PC is first turned on, so the +12 voltage is not overloaded by everything drawing maximum current at the same time. See this section for more on hard disk power issues and how problems can be avoided at startup.
Next: Redundant Power Supplies