Dell XPS 15 (L502X, Early 2011) - Manual (Page 85)

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Restoring Your Operating System
8. Select the check box to confirm that you
want to continue to reformat the hard
drive and restore the system software to
the factory condition, then click Next.
The restore process begins and may
take five or more minutes to complete.
A message appears when the operating
system and factory‑installed applications
have been restored to factory condition.
9. Click Finish to restart the computer.