Fill 0% on the layers.
Blending Options-Gradient Overlay

Create Microsoft Windows Vista Wallpaper in Photoshop CS

Gradient?s parameters:

Create Microsoft Windows Vista Wallpaper in Photoshop CS

Make three copies of the new layer and make the copies smaller again,
applying the Free Transform option.

Create Microsoft Windows Vista Wallpaper in Photoshop CS

Use the last instrument for creating one more layer.

Create Microsoft Windows Vista Wallpaper in Photoshop CS

Create Microsoft Windows Vista Wallpaper in Photoshop CS