With BlueStacks, you can run your favorite Android Apps on your Windows computer. Although a number of apps have their Windows version, some other apps are only available on Android. One example is WhatsApp.*
By installing BlueStacks, you can run Android apps as a "window" on your Windows PC. You can also make it full screen, setup Play Store and install other Android apps on it. It uses resources of your PC, which means Android apps and run faster, have more RAM and storage resources to run your apps. For some apps, like simulation games, this is important.
To download the free beta version, go there - http://www.bluestacks.com/. Although it is a beta version, it runs quite stable on my Windows 7, really worth a try!
*To install WhatsApp, you need to provide your phone number for activation, which will not work on a normal PC; but, after the activation timeout, then you still have an alternate method to activate it. However, note that your WhatsApp can only be installed on ONE machine.