How can I run multiple operating systems on one computer?

Below link is the guide on how you can run a multiple os on1 computer.
But I suggest not to as it will just slows down your pc.
http://www.wikihow.com/Install-Two-Operating-Systems-on-One-Computer

save from the different partition of your drive,,

Each operating system will be saved in a different partition of the hard disk and activated by going to dos command.
Better not to do this as your system may slow down.
Why do you need to use different operating systems?