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Media Player only allows you to switch regions a few times, and you must have exceeded your allotment. Even if you uninstall/reinstall, it won't allow you to do it again. If you want to watch those Japanese DVDs on your PC, you basically have three options. You could flash the firmware for the DVD drive, but I think that would cause the same problem for your region 1 DVDs. There are some software solutions out there, but I haven't tried most of them myself (I'm sure a helpful spammer will come along hawking their wares shortly). VLC Player will actually play other region DVDs sometimes, and sometimes not. It might be worth a shot. You can install a second DVD reader and SATA cable and use it exclusively for Region 2 DVDs, which is probably the best option and should cost about what one of those import DVDs is worth. I would go with the second drive option after giving VLC a try.
BTW, I highly recommend Sgt. Frog if you haven't seen it already. It's the funniest anime I've seen in a long time, and kid friendly so you can watch it with anyone. Samurai Champloo is also really good, but not kid friendly.