Genymotion. It's a fairly large download (more than 100MB) since it includes Oracle's VM VirtualBox. After installing it, start the application and it will ask you which Android device you want to emulate. Selecting the Nexus S might be a wise choice since it uses the least system resources. If you've registered at Genymotion, you also can choose other device images such as the Xperia Z. These images are quite large so it may take a while to download.
After choosing your poison, I mean Android image, you'll be greeted with the familiar Jelly Bean interface. Here you'll have to log in with a Google account so that you can download apps from the Play Store. Search for WhatsApp from your (virtual) Android device and install it. You may want to enter a few names in the contacts app with numbers of your acquaintances who you want to communicate with through WhatsApp.
To register WhatsApp you'll need to give a phone number (if you use your current WhatsApp number, you won't be able to use the app on that phone anymore since one number can only be used for one device). Better use a new number, which also can be land line number and does not necessarily have to be a mobile number, if you use that, an automated voice will call and tell you the WhatsApp registration code instead of receiving the code by sms.
Now you're all set to communicate with WhatsApp on your PC. Use only the virtualbox which came with Genymotion though. I made the mistake of upgrading to a newer virtualbox when it asked me and now the Genymotion doesn't want to start.