Many small appliances are designed to run on 120 V AC. Inside the computer, the 120 V AC is converted back to DC, giving voltages of +/-12 and +/5. It is terribly inefficient to convert 12 V DC to 120 V AC, then back again. It makes much more sense to use a bank of 6 V batteries (or 12 V batteries with regulators) to derive the required voltages and wire them directly into the computer (or other appliance). For other appliances, this voltage will typically be 6, 9, or 12 volts. While you're stocking up on batteries, run down to your local electronics outlet and stock up on voltage regulators. Virtually any device that runs on 5 V DC will work fine on a 6 V battery. Alternatively, if the level is critical, a diode or two in series with the power supply will drop a nearly a volt each, leaving a usable voltage level applied to the appliance.
Offered by Morgan.
All PC's are in fact DC. They run off a stabilized supply transformed from AC to DC. You should be able to connect a DC supply directly to the 12/9/5 Volt input power supply on the motherboard with little modification. Hard drives, CD's, floppies and monitors could prove problematic depending on type and input voltage, but on the whole they are transformed from AC to DC via internal transformers. This should also apply to most modern electronic equipment available today, however maintaining sufficient current on the heavier amperage devices is another problem.
Offered by Brian.
Although Im not 100% sure, Id imagine that most motherboards utilize DC. It is the computers power supply that converts AC to DC. Instead of looking for a DC computer package from a big suppliers like dell, Id look for DC power supplies that can fit any machine. Here's a Resource for you to start with. If you've ever installed an add-on card in your computer, installing a new power supply should not be too difficult. Screw it in place, affix power switch, attach special cable to motherboard, and attach four pronged cables to any devices that need power (CD-ROM, hard-drive, floppy, etc.).
Offered by Gabe.