Dell OptiPlex 760 - Manual (Page 31)

2. When the DELL™ logo appears, press <F12> immediately.
If you wait too long and the operating system logo appears, continue to wait until you see the Microsoft® Windows® desktop. Then shut down your
computer and try again.
3. Select System Setup.
4. Select Security® Chassis Intrusion and then select the Clear Intrusion Warning option to reset the chassis intrusion detector. Change the setting to
On, On-Silent, or Disabled.
5. Save your BIOS settings and exit system setup.
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NOTE: The default setting is On-Silent.