Is this your board?
it seems there is not just one P5B but the basic P5B & also several variations
Screw hole near the red IDE connector / Blue (USB) Connector
looks like its very close to the southbridge and, although not impossible, it's improbable that your PCI-e card has anything to do with the problem. PCI-e would normally be running off the Northbridge.
Check your box for anything that might have been able to short out to your motherboard. Maybe a screw got overlooked when building, thought lost .. then forgotten but waiting to take revenge ..