Some motherboards have a special switch that connects the case to a couple of jumper pins on motherboard. It is not usual in home PC's but in institutions, schools, offices and the such. It is a security feature to prevent users from messing with the system internals. If you have taken a MB from such a system and are using it in a non enabled case you will get this error. Otherwise you would get it by trying to use a system like this with its case open. If you can identify the switch on the MB you can jury rig a jumper wire or something similar to close the switch. If you can do this the error will stop. You may have to reset the bios if this MB has a lock out feature when the security switch is violated.