Sometimes you can plug a Japanese keyboard into an English system and it just works, but when it doesn't, try these steps.

1. Go to the Control Panel.
2. Open Keyboards.
3. On the Driver t, choose Update Driver.
4. Tell Windows Update "Not this time."
5. Select "Install from a specific location (Advanced)."
6. Select "Don't search."
7. Uncheck "Show compatible hardware."
8. Locate "Standard keyboards" under Manufacturer and choose "Japanese PS/2 Keyboard (106/109 Key)."
9. Click yes to the warning about incompatibility.
10. Click yes to the warning about replacing the PS/2 mouse port driver.
11. Reboot.