Let me get this straight: you put the Win7 distribution disk in your DVD drive, boot from it, and get that error? You are not talking about an OS on the hard drive?
My very first suggestion is that you carefully check your boot sequence to ensure that you are booting from the DVD. Go into BIOS, if you are familiar with it, and make sure that the DVD is on the list of boot drives, ahead of any hard drives.
When booting, look for a message like "Press any key to boot from CD..." - my system will bypass the DVD boot if I don't press a key. Don't bother looking for the "Any" key; Enter will work just fine (wink).
Without more info, it seems most likely that your system is not set to boot from the DVD drive, or that it can't see the drive. Less likely is that the DVD itself is damaged.
Write back; let us know what your level of comfort in BIOS is and little details like what kind of computer (if prebuilt) or motherboard (if you built it) you are using, plus any results you can give from my questions above. Then you can get further help.