I assume this means you're trying to boot from the Windows 7 DVD? The most likely cause is that your BIOS is not set to allow booting from the DVD. Go into the BIOS menu and change your boot device options so that booting from the DVD is enabled.
You can bring up the BIOS configuration screen by pressing a key (typically DEL or F2, etc.) right after turning the machine on while it's going through the BIOS self-check.