Just like Oracle software licensing, Microsoft has its complexities as well.
When using a Microsoft application, you must license each device in which you will access the software (locally and remotely). You can install any version of the application you prefer, for example many users currently will use Office 2003 instead of the newer 2007 version, but if both are installed, 2007 must be licensed. The same goes for a server installation – if computers in your office are using 2007, the server in which may house a remote connection must also be licensed for 2007.
For Operating Systems, you will need a license for each device that accesses the software but can only install one copy on each device (versus however many copies of Microsoft applications you want you may install). When we look at Volume licensing however, these are considered by Microsoft as an upgrade license. More information on that can be found here, along with a list of qualifying operating systems, which you must already have licensed in order to acquire an upgrade license.