Parallels Desktop for Mac does not include Microsoft Windows. If you don't have a Windows installation disc or .iso disk image, you can purchase Windows on a DVD disc from many retail stores that sell software or online from retailers such as Amazon.com or the Microsoft Online Store.
For Windows 8 or 8.1, you can use either the Full Product purchased at retail, or you can purchase the Windows 8 or 8.1 OEM System Builder version and use it in accordance with Microsoft Personal Use Rights. Please be advised that if you choose Windows 8 or 8.1 OEM System Builder Personal Use Rights, you will receive limited support from Microsoft as set forth here:
For earlier versions of Windows, you should purchase the Full Product from a Microsoft authorized retailer.
The list of all compatible Windows versions is available on this page:
Detailed instructions on how to install Windows into Parallels Desktop can be found in this article: