Did you burn the DVD yourself? If so, verify DVD against ISO using ImgBurn - making sure ISO when loaded says "bootable" in text at left, then verifying against DVD.
If necessary burn another DVD using ImgBurn at 4x with Verify - it never fails.
If you used the DVD successfully before, then check its condition.
after you insert you Win 7 DVD,
reboot your computer and make sure that your computer booting from CD/DVD as 1st preference.
you can check this setting in your BIOS.
after it is load your DVD just follow the instruction