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.