Do you have the chassis intrusion jumper placed like the diagram below shows, on the two right pins? If so, I’d examine the jumper closely to make sure it isn’t defective. If everything looks good, replace the jumper on the proper pins.
Go into the BIOS, look under the “Boot” menu, and access the “Security” settings. Make sure that the “User Access Level” option there is set to ‘Full Access’ so all BIOS options will be shown. If it was not, set it to “Full Access”, then exit while saving the BIOS setting changes. Try to power cycle the motherboard and re-enter the BIOS. Check see if there is now a ‘chassis intrusion’ setting under the “Boot” or “Security” menu that you can use to disable it.
If that doesn’t work, shut down and unplug the system from the wall. Since this board is so old, I presume you have replaced the CMOS backup battery on the motherboard with a fresh one?
- Remove the CMOS backup battery from the motherboard for at least 5 minutes or so
- Then, move the motherboard CMOS reset jumper from pins 1-2 to pins 2-3 for 10 or 15 seconds to clear the existing BIOS settings (and chassis intrusion alerts) from the CMOS memory.
- Replace the motherboard CMOS reset jumper on pins 1-2. Never try to power the system with the CMOS reset jumper on pins 2-3 or it will almost certainly damage the motherboard.
- Replace the CMOS battery back into the motherboard
- Plug the system back in to the wall and try to start it back up
Hopefully, this will clear the intrusion alert and let you boot normally without throwing the error.
If it continues to throw the same error after clearing the CMOS memory, try to troubleshoot it as an electrical short or ground fault between the motherboard and the case. You would do this by removing the motherboard from the case and placing it on a piece of cardboard, followed by hooking the PSU power cables and monitor back up (if it doesn’t have onboard video, plug in the graphics card and hook the monitor up to it). Then, try to boot it there on the cardboard. If it still throws the error, try clearing the CMOS memory again, followed by powering it up and trying to start it yet again.
If you continue to get the error after doing all this, it is possible that the motherboard is damaged in some way and may never boot up. As a last gasp effort, if the POST screen actually displays before the error is thrown, you can try to download the most recent non-BETA BIOS from the ASUS website linked below, unzip it and place the BIOS file on a 3 1/2 floppy disk, connect the floppy disk to the system, power it on, and press ALT-F2 at the POST screen. The system should then automatically try to flash the BIOS file from the floppy drive.
EDIT: ASUS still has a support website for the board here with BIOS and drivers.