An old laptop I found (an an Acer Aspire 5100 series BL51) gives this error:

Phoenix TrustedCore(tm) NB
Copyright 1985-2005 Phoenix Technologies
All Rights Reserved
System BIOS Version: V2.00
Bus:06, Device:02, Function:00
ERROR
Resource Conflict - PCI in slot 03
Bus:06, Device:04, Function:00
ERROR
Resource Conflict - PCI in slot 03
Bus:06, Device:04, Function:01
ERROR
Resource Conflict - PCI in slot 03
Bus:06, Device:04, Function:02
ERROR
Resource Conflict - PCI in slot 03
Bus:06, Device:04, Function:03
Based off of that error, I'm assuming there is a problem with PCI in slot 03, which I'm also assuming the item inserted into the PCI slot 03, but I cannot find which slot is which because they're not labelled or anything. I've tried looking on the Acer website but most things 404 and a PDF of the manual I found has nothing useful at all.
Without the manual, how can I find the problematic PCI slot and fix the issue?