You can re-install Windows 7 (or a different OS over Windows 7) by mounting an ISO disc file to your ROM drive or by placing the Windows 7 (or a different OS) installation disc into your physical ROM drive. When you install Windows over Windows it will create a Windows.old folder in your C drive directory which contains all of your old files from your previous installation.
Here are the list of operating system according to the maker: Microsoft Windows NT 3.1 Windows NT 3.5 Windows NT 3.51 Windows NT 4.0 Windows 98 Windows ME Windows 2000 Windows XP Windows Server 2003 Windows Vista Windows Server 2008 Windows 7 Windows Server 2008 R2 Windows 8 Windows Server 2012 Mac Mac OS X 10.0 Cheetah Mac OS X 10.1 Puma Mac OS X 10.2 Jaguar Mac OS X 10.3 Panther Mac OS X…
There are several Mac OS X themes or OS X inspired themes available for Windows 7. Two of the popular ones are Snow Leopard by giannisgx89 and Leopard Mod by sagorpirbd. These themes or visual styles don't require any additional software. However, the do require the Windows 7 themes subsystem to be patched. Refer to the related links section for link to UxStyle - a simple and safe patch for Windows 7
Snow Leopard and Windows 7 are two completely different types of operating system. You cannot use software written for Mac OS X (Snow Leopard) on a computer using Windows 7 (or vice versa). On a Mac you can run both operating systems so you can run Windows 7 instead of Mac OS X or in a separate window on top of Mac OS X (Snow Leopard) using Parallels or Fusion (See links below).