Different slot type entirely there. You will always find pci slots on any old system while many lacked the AGP type until just before the first Pentiums/Pentium Pro were ouf(I286 to I486 time frame) and simply used the pci type as the standard addon type card. AGP later offered the improved faster bus over the ISA to PCI progress there.
If the board there is equipped with an AGP slot that will be slightly different in appearance to the standard pci type slots it is just above. Look at the PCI-E type 1x slot on a new board to see how there;s a slight gap seen between that type and the 2 pci slots typically seen on a new board. The slot itself will only be slightly shorted in liength then the 16x slow however commonly in use.
Any expansion type device added onto a system requires the device drivers needed for the particular OS being run on it in order to see it funtion fully. Windows includes it's onw basic vga type drivers to allow you to operate a system with a low resolution screen until all drivers/softwares are installed.