Do you get a prompt to "Press any key to boot disk" when you set DVD drive first to boot in BIOS setup?
If not try triggering DVD with the one-time BIOS boot menu key given on first boot screen, normally ESC or F9 on HP's.
If it doesn't boot then there's something wrong with the DVD, ISO or burn. Is DVD new, clean and unscratched?
If in doubt start over from Steps 1 and 2 from Clean Reinstall - Factory OEM Windows 7.
If DVD method fails then try the USB install method given. If that fails unplug the DVD drive and try again.