In all likelihood, all your files are on the same disk partition as your old operating system. You'll need to install your new OS onto a new partition. If you have a large flash drive or another hard drive, I'd recommend transferring all the files on your hard drive (other than the old OS) onto said storage medium and then installing your new operating system onto your hard drive. To do this:
1. Upon your system booting, go into the BIOS and reorder the boot priority of devices so that your optical drive is first on the list.
2. Shut down the system, insert the new Windows disk into the disk drive, and start the computer again.
3. Follow the installation process. (If you have transferred all of your files from this hard drive to another drive, select "Clean Install." This will wipe your hard drive before installing your new operating system onto it; this is good, as it frees up valuable space, but only if there isn't anything on the drive that you need.)