"Restart the computer. To do this, click Start, click the arrow next to the Lock button, and then click Restart.
As the computer restarts, press the F8 key until the Advanced Boot Options menu appears on the screen.
Press the (Down Arrow) to select Repair Your Computer on the Advanced Boot Options menu, and then press Enter.
Specify the language settings that you want, and then click Next.
Log in as a user who has administrative credentials, and then click OK.
Click Dell Factory Image Restore.
In the Dell Factory Image Restore window, click Next.
Click to select the check box for Yes, reformat hard drive and restore system software to factory condition.
Click Next. The computer is restored to the default factory configuration.
When the restore operation is completed, click Finish to restart the computer."
I hope this helps.