Licensing affects all operating systems and applications. With VirtualBox, this OSE (open source edition) can be freely distributed and used on any computer. However, with Microsoft the end user license agreement (EULA) is affected by how and where the operating system is installed.
Although VirtualBox is free, Microsoft licensing still applies to virtual computers. Both server and client operating sytems (Windows XP, Windows Vista, Windows 7, Server 2000, 2003 and Windows server 2008) all must have their own license. The EULA or end user license agreement provided by Microsoft still applies to virtual computers.
With servers, the server's edition may have restrictions. Datacenter editions are not truly limited whereas other editions Microsoft allows for the physical computer (machine / host) and four virtualized computers. Microsoft Server 2008 allows for a physical and virtual instance.
Most Linux editions allow for as many virtualized computers as the host will handle. This of course is based on the resources of the host computer (memory, CPU and hard drive space).
Home users do not have to purchase a VirtualBox license, but commercial users should use and purchase a commercial license. With VirtualBox OSE (Open Source Edition, the software is released under the GNU General Public License V2. This release means that you can distribute and modify the software as longs as you ship the modifications under the GPL (General Public License).
Ensuring Licensing Recommendations are Met
Home users should understand that any copy of Microsoft Windows should only be installed on one computer. Any virtualized computer is considered a computer. With Microsoft Windows Vista, the original shipping of the operating system did not provide for virtualization of the operating system. After the release of Service Pack 1, Microsoft Windows Vista is now covered under virtualization. Following the EULA is important for home users as well as commercial users.
If you have an older operating system disk and VirtualBox, you can use the older operating system as a virtual computer. Virtualization and virtual computing are the new way of using computers. With hardware becoming more economically feasible, resources (memory, CPU and hard drives) are faster, bigger and more abundant. Home users can now use more than one operating system on their computer. Sun Micro's VirtualBox can now be used to run older programs or network applications for computers in a local are network. Although virtualization is being used in commercial business, many home users can now begin using applications like VirtualBox.