If the dell has be setup corrcly it should have a recovery partition o the hard drive. To acsess this turn the laptop on from cold tap f8 untill you get a list of options lile repair your computer. Select repair your computer then wait a minute and when promped ener your windows user name and password. Next screen select dell image/factory restore. Then give ot an hour to restore
Id there is not dell image restore option then yes find the dell os disc that should cool with
Good luck