Well, WhatsApp just released a web client but using it is a bit a pain and isn't really worth all the trouble.
- To set it up, you would need to have Google Chrome installed on your PC.
- Then you need to have this version of WhatsApp installed but first you should do make a backup of your messages, then uninstall the WhatsApp you have before installing this version. Download WhatsApp
- After installing the app launch it and set it up like you would normally do.
- Open the web version on your desktop using this link http://web.whatsapp.com
- Open the WhatsApp app, press the MENU option and select WhatsApp Web
- Use the QR Scanner to scan the QR code on the Web Browser and you are good to go.