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.
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 also a freely distributed program where you can install all 9 different operating system from windows and start switching between them.