Change the settings to unhide any hidden files and folders and see if you have a Recover Partition. If you do then inside it should be a WIM file (unless Dell use Ghost). If there is a WIM file you can copy that to a USB (make it bootable and Google DISM, Deployment Image Servicing and Management. It is a Microsoft tool that can be used to "write" the WIM file back to the HDD and return it to Factory Condition).
Alternatively, obtain some Dell media (eBay) and use the Windows 7 key that should be stuck to your machine. If it's a laptop look in the battery well. (They seem to have stopped putting the key sticker on Win 8 installs now) Another possible option is to get a copy of the same version of your operating system and install that but use YOUR key from your machine though I have found this a bit hit and miss.