As others have said, Apple has a very strong presence in Japan. If I go to an electronics store, usually half the models on display are Apple. Panasonic and Fujitsu pretty much rule the desktop with bizarre television/computer fusions. Imagine a TV about the thickness of one of the old plasma TVs. It usually has a stand, but I think you can also wall mount it. You can watch and record TV on them, so they are basically consumer devices.
For laptops, the biggest brand (other than apple) is probably Asus (based on retail floor space ;-) ). Sony, Panasonic, Fujitsu and Toshiba pretty much share the rest. Toshiba is apparently exiting the market soon, though. From my perspective, it seems that the market is split between very high end, large format, desktop-replacement machines and tiny machines. There is not very much in the middle. I live in the countryside, though. It is very possible I don't see the interesting models where I live.
When I worked at a high school, all of the machines were actually Dell. It surprised me greatly.
One thing that most people don't realise is that Japanese electronics are expensive compared to the rest of the world. If you have high end purchases (like a computer), you can fly to Korea and the money you save will pay for the flight, easily. Especially with the insanely high yen, American machines are also a pretty good deal.
I like to support the local economy, but when Toshiba exits, I might be stuck finding the kind of machine I like to use.