So far as I know, your power supply is the only component that you have to worry about being compatible with your country's power grid. (Well, and the monitor and anything else that you actually plug into the wall.) Once inside the case, the power is converted from AC to DC and is all standard, regardless of country.
The power supply is indeed all you have to worry about. The other components don't care if the power-supply inputs are 115V AC, 220V AC, or even DC, as long as the power supply works.
Most AC power supplies have a little red recessed switch near the power input socket that's labeled for "115" or "220". Just flip it to the appropriate position. <b>DO NOT, i repeat, DO NOT</b> plug such a power supply into a 220V circuit unless it's switched to 220V--it will blow the power supply and possibly other stuff in your system!
If the possibility of making such a mistake scares you, simply get a power supply with auto-switch capability--it will automatically detect the A/C input voltage and switch itself accordingly. <A HREF="http://www.pcpowercooling.com/" target="_new">PC Power & Cooling</A> sells very good power supplies, and you can specifically request an auto-switch feature for a small price (maybe US $5-10).
<i>I can love my fellow man...but I'm damned if I'll love yours.</i>
Processor core runs at 1.8v or less, depending on type (at least anything recent). Mobo uses 3.3v, 5v, and 12v signals for power and -12v and -5v for switching. Graphics cards use 3.3v or 1.5v depending on type. CDRW has a 12v and 5v line running to it. Same with hard drive. All currents being DC.
So what does that leave to convert all the voltages to 3.3v, 5v, 12v, -5, and -12v? The power supply. It has to make the current DC as well, so output frequency doesn't exist. Most decent power supplies have a switch on the back for 120v or 240v operation (still called 110/220 or 115/230 by some companies).
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