Note to get the recovery options mentioned, power down your Dell. Then power it back up and tap F8 repeatedly. If Dell DataSafe Restore and Emergency Backup does not display there has been some changes to your internal factory partitions or an Operating System has been manually installed. In this case you should restore from the recovery USB Flash Drive see:
How to Restore using USB Recovery
All of these options do not allow for clean installation and all the Dell preinstalled software and drivers are loaded, if you prefer a clean installation or cannot procure recovery media see Windows Reinstallation Guide/A Clean install of Windows 7.