In BIOS there is usually a list of three devices to boot from, in priority of 1, then 2, then #3. If your CD or DVD drive is listed as the first boot priority, then usually your hard drive is listed as second. That way, when you turn on your PC and there is no disk in the optical drive, the BIOS moves on to the second boot priority device, the hard drive, and boots from that. This setup will delay each boot of your PC by a few seconds, so most people change the first boot device to the hard drive once they install Windows.
The only way for you to screw up your PC booting is if you selected the DVD drive to be the boot option for all three priorities, as in first priority, second priority, and then again #3. That means that the BIOS...