Try entering your BIOS using the Del, F1 or F10 keys, and set the first boot device to the one you are trying to use to install the OS - usually the CD/DVD drive.
It seems as if you don't have your DVD drive as the first boot device.
Do as treefrog07 said, and go into your bois. There you should be able to set your DVD drive as the first boot device. Once you do that and have windows 7 installed, remember to set the boot settings to how there were before if you want.