I think Acklan's on the right track - if you had a PCI card, there may also be an AGP slot, but neither is your onboard video. So when you said,
So after I switch to AGP in the BIOS
. . . it makes me think that you either have a third choice, or you're doing something else wrong. If you have an AGP card and you've been using a PCI card, it's usually normal to have one or the other but not both and onboard video means "on the motherboard" - not in the AGP slot.