Hello,
first of all do you mean Whatsapp for PC, if yes then here is a bit explanation.
For downloading All you need is Bluestacks as colleen had mentioned above, but that as well some times give error..
Reason 1: Download Broke up ( Re download it and it might solve the issue)
Reason 2: Your PC is without Graphics Gard or with low memory ( Please not to installl Bluestacks you need Good Ram and Graphics Card - Still Solutions is Available)
Reason 3: Instead of windows version you might have downloaded Mac version...
and so on what i know at present.
All i have found is a interesting post with a video tutorials as well a download link if you have Graphics issue... Download and Install Whatsapp for PC
I have found that article a nice write up, even through whatsapp download is pretty easy just download the correct download file, instead of not reading and downloading a wrong on....
Hope all your doubts are clear now...