Perhaps the easiest way to change the look of your PC is to pick a new wallpaper image for your desktop background. If you're bored of the standard images supplied with Windows, there are many sites from which you can download others. Wallpoper, at www.wallpoper.com, is one of them. It's free, and offers a choice of 350,000 images to search from.
And if you're wondering why all the search results in the screen shot below are the same colour, I shall tell you. The search system allows you to search by colour as well as by keyword. Neat.