Slow down...
So you have installed the Japanese version of Vista over your English version of XP? :O
I'm not familiar with Vista, but I can try and tell you where you might go;
firstly you might wanna go to Control Panel / コントロールパネル、
after that, look for System / システム
then in the tabs look for System Restore / システムの復元
Like I said I'm not familiar with Vista, or the procedure for restoring back to XP, but that might be worth a try at least.
ガンバレ
