I have Win 7 64bit which i payed 100$ for and i also want linux for security. Is there a way that on bootup i could choose which OS I want to run? Also will all of my documents and files stay saved on both OS's including my Windows Games?
Yes, you can configure GRUB to hand the bootloader to windows even if it is on another drive. This is especially easy on ubuntu based distro's because it has a special script that does this auto-magically. 'update-grub'.
Yes all your files will stay on the windows drive/partition.
You can access the windows file systems from linux (you can mount them as read only or with write permissions). You will not, by default, be able to access linux file systems from windows. There are some work arounds like installing a third party driver in windows to read the linux ext partitions or simply by having a common NTFS or FAT partition to share files between OS's.
If you are new to linux consider looking at:
Ubuntu, mint, any of their derivatives, or fedora. These will probably be the easiest for you to get started with.