Virtual Operating System (OS) is a concept of having more than one Operating System on your single PC. If you want to install Windows as well as Linux on your PC, then go for the virtual OS where you can switch between the two OS on a single PC. This is very much essential for the developers to test the application on different OS by using a single PC.
List of Software to create Virtual Operating System on Windows
This is a product from Microsoft and it is free to download as well. Here you can switch between different Windows (included 2003 Server, XP, Vista editions, XP tablet PC etc)
This is a paid application where you can switch between windows as well as Linux operating system.
This is a platform where you will get everything on Virtual machine. Whether you need a virtual Datacenter, virtual server or desktop, you will get here.
It is an X86 virtualization package which is now maintained by Sun Microsystems as part of Sun xVM platform.
This is also a freely distributed program where you can install all 9 different operating system from windows and start switching between them.