Following my post from yesterday, this may be an even easier way to ensure that you are always playing notes from the right key.

CodeFN42 have released a FREE VST plugin called Cales.

Cales is a VST plugin that allows you to map your keyboard’s keys to a musical scale. This will ensure that you will always stay in key, and never hit a wrong note.

When "easy mode" is enabled, you can play any scale using only the white keys with "C" always being the tonic (or "root" note).

More than 20 scales are included, and you can easily add your own custom scales.
Get Cales here.