If you use Windows but love simple, minimal and clean look of Ubuntu, here is something interesting for you.
"Ubuntu Skin Pack" is a customization pack for Windows 7 users which can make our Windows look-like Ubuntu 11.04.
This pack changes almost everything in Windows 7 to transform it into Ubuntu. It changes login screen, boot screen, wallpaper, theme, start button, icons, etc. It also installs a few utilities to provide Ubuntu functionality.
NOTE: Please create a system restore point before installing this skin pack so that you can restore default Windows look in case you don't like the new look or face any problem.