The following is from the Setup Guide from Dell Support:
Dell Factory Image Restore
1. Turn on the computer. When the Dell logo
appears, press <F8> several times to access
the Vista Advanced Boot Options Window.
2. Select Repair Your Computer.
3. The System Recovery Options window
4. Select a keyboard layout and click Next.
5. To access the recovery options, log on as a
local user. To access the command prompt,
type administrator in the User name
field, then click OK.
6. Click Dell Factory Image Restore. The Dell
Factory Image Restore welcome screen
NOTE: Depending upon your configuration,
you may need to select Dell Factory Tools,
then Dell Factory Image Restore.
7. Click Next. The Confirm Data Deletion
NOTICE: If you do not want to proceed with
Factory Image Restore, click Cancel.
8. Click the checkbox to confirm that you want
to continue reformatting the hard drive and
restoring 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 reboot the system