event id 2 Chassis intrusion detected

I’m getting the following error in my event viewer:
event id: 2
Chassis intrusion detected. This typically does NOT indicate a hardware failure.
1. Contact your Help Desk if you did not personally open your chassis.
2. Enter System Setup & Set Chassis Intrusion to ‘Clear’.
Any one know how to set Chassis intrusion to clear ? and what this means ? the user did add a Compaq ipaq 3835 PDA to this machine.