How we put 2 system operation in our computer???

I think dual-boot is the best option. Live-CDs aren't very cool cause they depend on the speed of your CD/DVD drive. Mr mlk has given a great detailed explanation of the dual booting procedure. If you don't know how to get your computer to boot from a CD-ROM then you'll have to go into your BIOS and set the boot-sequence in such an order: first boot device: CD-ROM second boot device: Hard Disk and so on.... If you don't know how to get into the BIOS then check youe motherboard manual.