If your operating system is VISTA / Win 7, press the F8 key when you see the Dell logo. It will take you to the advanced boot menu options, which include 'Repair your Computer'. Select that option and enter your password if asked, then you will see a list of options like 'system restore' and 'command prompt'. The last option says 'Dell PC restore', or something along those lines. Select that option and follow the instructions on the screen.
If in both cases, the Factory default doesn't work, then your laptop's factory image defaults partition has vanished (i.e., been corrupted / deleted). In this case you will have to reinstall the operating system with the CD or DVD shipped along with the laptop.