If Windows 8 was installed with GPT partition style (when UEFI is present, this is the only option supported), all the partitions will be removed. Hence, in this case, you need to contact Dell support over phone, explain the situation and request a new Windows 7 DVD. If Windows 8 was not installed in GPT partition style (i.e in MBR style), and the recovery partition was not disturbed, you can start Dell Recovery by pressing F11 or Ctrl+F11 when you see Dell logo during boot, and it'll restore the PC back to Windows 7.
I have purchased a Dell Latitude 3540 with Windows 7 Pro.  I installed Windows 8 with the recovery media, but I would like to recover my Windows 7 installation.  Thank you for your time.
How to recover Windows 7 on Latitude 3540