You wont believe that you can change windows 7 appearance totally with some Themes. Many might not be interested to know about it but see an image of my windows 7 now.
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Now you would like to know how to the Windows. Its quite simple. First your windows must recognice the themes you paste, For that you will have to download a small software called Universal Theme Patcher.
Now starting with from where to download your themes, Deviantart is currently the best site to download themes for windows 7.
Download any theme you like and always remember that the theme will not be a self install theme. You will have to install the theme manually. If you want a theme try visiting here.The theme which you have downloaded will be a zip or rar[rarely]file. You will need winrar to extract it. If you dont have it download it. After extraction you will have two required files[generally]. Go to C drive in it to the folder windows then search for the folder resources and go in it.
Paste the two files in the folder themes. If you want to open this theme just go to desktop , right click and select personalise as you know to go to the themes menu. You can find the new theme here.
Here are some themes