The keyboard idea would be easy with a PS/2 keyboard and an HID portal or AVR-USB on the Arduino (though AVR-USB on Arduino isn't easy yet). If you wanted to use a USB keyboard, it's still possible but harder; I'd go for the Vinculum solution from FTDI. But yeah, in XP you can easily select a Dvorak keyboard layout.
The Arduino would work well for the aquarium alarm. You could use the Ethernet Shield to get the Arduino on the internet.