Place the CD into the optical drive and reboot your PC. If the install does not start you can try either of the two following during the beginning of the boot sequence.
1. hit F8 (or similar key depending on PC) and choose the optical drive too boot.
2. hit Delete key (or similar, depending on PC), go into bios and boot settings, set your optical as main boot device then HD as second.
Read carefully during the install process and you should be fine.