I'm currently learning XNA per suggestion from this question's accepted answer:
Where to start writing games, any tutorials or the like?

I have then installed everything to get ready to work with XNA Game Studio 4.0.

General Objective

Writing an Arkanoid-like game. I want to make my ship move when I press either left or right keys.

Code Sample

    protected override void Update(GameTime gameTime) {
        // Allows the game to exit
        if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
            this.Exit();

        // TODO: Add your update logic here
#if WINDOWS
        if (Keyboard.GetState().IsKeyDown(Keys.Escape))
            this.Exit();
        else {
            if (Keyboard.GetState().IsKeyDown(Keys.Left))
                MoveLeft(gameTime);
        }
#endif

        // Move the sprite around.
        BounceEnergyBall(gameTime);

        base.Update(gameTime);
    }

    void MoveLeft(GameTime gameTime) {
        // I'm not sure how to play with the Vector2 object and its position here!...
        _vausSpacecraftPos /= _vausSpacecraftSpeed.X; // This line makes the spacecraft move diagnol-top-left.
    }

Question

What formula shall I use, or what algorithm shall I consider to make my spaceship move as expected left and right properly?

Thanks for your thoughts! Any clue will be appreciated. I am on my learning curve though I have years of development behind me (already)!