Not sure what exactly you have tried yet.
You could try these until we can get more info on what you have tried.
Dell -- Older Dells came with CDs for the OEM version of Windows, drivers, and some of the installed applications. Starting in mid-2004, Dell has been shipping their laptops with a utility called Dell PC Restore by Symantec. This utility is on a partition on the hard drive and can be triggered by pressing Ctrl+F11 at the Dell BIOS startup screen for XP laptops, and by pressing F8 after the BIOS screen (like you would to get into Safe Mode) and choosing the system recovery option on Vista machines.
Or this link at Dell support may help.
Dell - Support