well CMOS/Bios menus are different from PC to PC but if you get to the cmos screen look for PCI configuration, it might be under the advanced tab.
Then look for something like "Primary VGA bios" and you should be able to set that to either AGP or PCI.
IT will probably default to agp if you have an agp card.
also look at this list of supported display adaptors
microsoft supported video adaptor list