Be aware that some older slot 1 boards did just that. An AGP had to be fitted and that was that. Very annoying board. Rare, but not unheard of. Some E-Machine from what I remember.
WHy not share the make/model?
Remember that the ISA card memory space is fixed and can interfere with a PCI video card.
Slot 1 650-mhz agp slot , but set to check PCI first in BIOS, no onboard video, old VGA ISA card works in ISA slot, 3 PCI video cards 8mb low end S3 type will not post, get beeps that indicate no video card, looking for help to get pci cards to work!