hello i have my laptop on which i've first installed win 7 and i whish to install linux centos on the second hard disk please how should i procede to get both os on my machine??? i am a new comer on linux platform

Welcome to the forum!!

Please note that "bad" is not a good thread title---it gives no information about what your question is...

Do you really have a laptop with 2 hard disks?----or perhaps you meant install on the 2nd partition?

The first thing to do is to re-size the Windows partition to make room for Linux. Most modern installers will include utilities to do this, but I always use GParted (comes on a bootable CD).

Once you have created the empty space, the installer will handle the rest. Have you already downloaded CentOS and made an install CD?

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Adding to @pixellany, first check availability of space on your H/D. If enough space (i.e. recommended space) is available, then make a seperate partition on your Windows H/D (using My computer > Manage > Disk Management > Make partiton), name it something like F or G drive, format it and then start your installation using a CD drive or a USB.
- Insert CD or USB
- Restart your PC.
- At boot menu, select CD or USB as your first boot device. Save & exit
- Installation will begin, after some steps, it will ask you to choose installation location. Here be highly careful and select the new partition which you have created i.e F or G for Linux installation. (else one mistake might distroy already installed Win7) and go ahead.
Keep in mind: In any case, before proceeding, take backup of all your data from Windows7. If you're not sure about the steps, or have no previous experience of such installation then backup is a must and first step.