Recent versions of Windows Server include Windows Server 2008, and the newer Windows Server 2012.  Your major software package may require one of these Windows Server Operating Systems.  Windows Server Operating Systems require licenses from Microsoft, or something they call CAL's (Client Access Licenses). The cost and number of CAL's you need to buy is based on things like the number of users who will be using the server.  Different features within the Windows Server Operating System can be turned on or off, and often require their own CAL's.  Just to illustrate the complexity of Microsoft Licensing, check out the following information from Microsoft Licensing CAL's. If that information does not give you a headache, you can then look at the pricing involved and start shopping!   These are all waters that we have navigated and are familiar with, based on servers we have installed.  We will save you a great deal of time and money by simply showing you what you need to buy and prevent you from making costly mistakes.  The major software application that you need may require that your server runs Microsoft Windows Server.