What you have to do is put in the Windows disc and set the dvd drive as the boot device and let it install windows. The message you are recieving means that the drive does not have an operating system on it so you need to load Windows on to it. If the computer can't find an operating system on any of the disks then that is the message you will get.
Your computer tries to boot from the hard disk instead of the Windows 7 install DVD.
You have two options.
First, watch carefully the splash screen when your computer starts. It indicates, what key you have to press on the keyboard to go into BIOS settings (Usually DEL, or ESC), and there's another key, (usually one of the function buttons, like F11) to go into a menu which lists the available drives for booting. Get into this menu, and select the DVD drive.
Second, you can set the boot device order in BIOS settings. If you're not sure about this, consult your manual, or let us know the exact type of your motherboard.
PS. I have to type faster if I want to beat these guys next time.