If you installed Windows 8 over Windows 7, the chances are you have wiped the Dell recovery partition.
Without that or a set of Dell recovery discs you won't be able to recover your system.
You have two choices as I see it:
Clean Install : Factory COA Activation Key
- Create your own Windows 7 DVD and install that using the product key on your COA sticker.
- Contact Dell and ask them for a set of recovery discs. If you tell them your recovery partition is corrupted they'll most likely send you a set without charge.