Shut down your computer
Power your computer back on, but be ready.
The BIOS screen is the motherboard boot screen. This is NOT the operating system loading, this takes place before that. Generally, it will be a screen with the computer manufacturer logo, and a little bit of text. You will need to press a certain key at this point, and it varies depending on the motherboard maker, BIOS, and computer manufacturer. Generally, it is ESC, F1, or DEL. To check what yours is, look on the bottom of the BIOS boot screen, and it should show something like this:
Down on the bottom, it should say "Press [key] to enter SETUP". Press that key, and let SETUP load.
After the BIOS SETUP loads, you should see a screen similar to THIS:
Scroll over to the BOOT menu using the Arrows Keys. It should automatically show the menu, but if not, then hit enter. This should bring up a menu like THIS:
This is where it varies depending on the motherboard. Some will show the menu as it is shown below in the image, and others will have a secondary menu for the Boot order.
Now just select where you want to boot from first, second, and third. It is simple if you follow the instructions in the BIOS screen.
After the new order is set, press F10 to SAVE AND EXIT, or scroll over to the EXIT and select save changes. This will REBOOT your computer.
The computer should now start up and load from the first boot device. If it is the CD/DVD Drive first, then it will appear to hang at a black screen with a white cursor.
If there is no CD/DVD, then it will eventually move on, be patient. If there is a CD/DVD in the drive, then it will say: "Press any key to boot from CD..." Hit the key to load from the menu if that's what you need. If not, let it Boot normally.
This procedure is used to change the BOOT order to allow a boot up from a CD/DVD or a different HDD. This is used to reinstall/install/repair an Operating System (i.e. Vista), or to boot from a different OS first.