WhatsApp is not officially released for desktop PCs, but there is a way to make it work by using an emulator. In this case, we'll use Bluestacks. Visit the application's page on Software Informer and click on the download button. Download Bluestacks and install it then launch the application on the desktop. Make sure that the computer is connected to Internet and the connection is stable. Wait for the application to load and use the built-in search to look for WhatsApp. Once you have found it, use the mouse and mark it to be installed. Once the process is finished, start the application and make sure that you have your phone by hand as you'll receive a confirmation PIN that needs to be entered into the confirmation box in Bluestacks. Since WhatsApp uses Internet to send messages, you won't need the phone anymore. Once the application is activated, you can use it normally as if it were in the phone.
P.S: Use the same built-in search function to look for a Contacts application. The reason for this is because WhatsApp uses the contacts stored in the phone to look for people using the application. Download the contacts related application and add people from your phone. Use the WhatsApp menu to scan for contacts then you can fully use it as if it were on a real phone.
This is the only way to make the application work on a PC desktop.
If you can't download WhatsApp, use the direct download URL to get the APK file from the official website. Save it to your desktop and double click it only after you've installed Bluestacks.