You have two options:
Use your PC as is with an external 110-to-220V transformer. Straightforward, but make sure the transformer's wattage is sufficient for your PC and that it's a high-quality model suitable for extended use. (Cheap, underpowered ones have a disturbing tendency to catch fire if overloaded for long periods of time.)
Replace your PC's transformerpower supply (PSU) with a Japanese model. Transformers Power supplies are cheap and fairly standardized, so this is pretty straightforward and shouldn't require anything more than a screwdriver, but do double-check the transformerphysical dimensions, output voltage and the type and number of output plugs to make sure the replacement is compatible.