Here is a small tutorial on how you c an use the viber on your computer:
As viber is a mobile application you need to get the smart hone mobile operating system emulator on your computer.Viber is available for the top mobile operating systems Android,iOS, Windows Mobile operating system, Blackberry Operating system and Bada operating system.
We shall get the viber's Android app as it can be easily played (run) on your pc with BlueStacks.
BlueStacks is a free Android Emulator you can get this from here.
Blue Stacks lets you to directly download the Android apps from the Google PlayStore in easy way.
In-order to install the Android Apps from Google Play Store you need to Connect/register the bluestacks with Google PlayStore with your Google Account.You can also create a new Google Account directly from BlueStacks and Connect with the Google PlayStore.
After completion of connecting your account with BlueStacks you can download the viber directly from Google Play Store.(If you have any problem you can download the app from other app stores).
After Installing the Viber on your computer. You can login into your account with your viber number and you can also create a new viber account from BlueStacks(You need to verify your mobile in both the cases while register from BlueStacks and register from any smartphone).
After completion of all the steps you can call and message to any viber no for free from your computer!.