When I started composing music through the computer I used a very simple music keyboard, it was called the evolution ekeys. It was a simple 39 mini keyboard which is USB, in windows XP it is simply plug and play, and if you are on a really limited budget then this is the music keyboard I suggest, it is however a little flimsy and not the best feel.However for a little bit more money (and we are not talking a lot) can buy the Ekeys 49, this music keyboard is full size, velocity sensitive with modulation, pitch bend,octave transpose, a sustain input and programming options and other features that I will never understand or use.What I liked most about this product was the feel, when you touch the keys the bounce is just right. It is sturdy, and not too heavy.
You can get some very cheap second hand midi keyboards, however make sure their input to your computer is USB not the joystick port, USB for midi does make life easier.Unless you have some very special needs, this music keyboard is perfect. 10/10