I'm assuming you will do this on the Vaio from your other post so I guess you could try the following if it has to be Whatsapp:
Download Bluestacks and install it. It should be supported by Win XP SP3 - So you might want to run Windows Update first to make sure you have all the updates installed for XP.
Check if sound is working.
Enable/log in to Google Play.
Download Whatsapp.
Download TextNow (This can be done one some other device as well, like your phone. If you already have an account and a TextNow number there that haven't been used for Whatsapp earlier you can just use that one. It is just to receive a phone call from Whatsapp to get the activation code. )
And basically just follow the rest of the steps that is in the
URL that
@Colif posted.
But also as mentioned before, bluestacks might be choppy on that Vaio, since it does not meet the minimum requirements for it.