Hi, first you'll need to amke sure you have room on the hard-drive.
From vista, use the defrag tool to consolidate your windows files.
When you are sure you have enough space, instert your centos (or suse) cd and reboot the PC.
There are plenty of howto's written about this, as I do not use vista
I recommend you search LQ for a tutorial/howto of dual booting vista and centos (or suse).
It can be done, but it may also be dangerous! Backup first!