You need to change your boot order in BIOS.
When your computer is first turned on, before windows starts, you should see a message that says to press a certain key to enter setup. It is usually an Fkey or esc or del. Pay attention, as the message may flash very quickly.
Once you get into setup, you want to look for BOOT ORDER or BOOT SEQUENCE or maybe even just BOOT. Set you CD drive as the first boot device and then place the CD in the master CD drive.
Now exit setup and SAVE changes.
When it starts back up, you may see a message that says: Press any key to boot to CD. Just keep tapping the space bar as the computer starts up.