There's no way to do this without the disk. You can contact Dell or the person that you bought the laptop from, for the restore disk.
Another option is to format and install from a new Windows O.S. disk. That's what I did with my Dell desktop pc. The Dell restore disk is filled with software that I don't use, so I opted to simply install Windows XP and leave off the rest.
My parents got me a used Dell Inspiron 5160 laptop but had no reboot or restore disc. How do I restore it to its original system?