Look at your board and make sure it has a physical pci-e slot,before you buy a card.The software says you have pci-e but it may be an onboard device using it,and not a physical slot.Best to be certain.
Smart man here, definitely look at the board and make sure it physically has a PCI-E slot. Even though the software detects a PCI-E Bus, you might not have a PCI-E graphics slot, it could be picking up a PCI-E x1 slot, or something onboard using the PCI-E Bus.
Or it could be that the Bus it here, and the connectors for the slot are on the board, but the physical slot is not actually there. A lot of pre-built manufactures do this.
Edit: Ghost101 already beat me to it. This is exactly what HP has done here, the PCI-E bus is on the board, but the phsysical slot is not. So software will pick up that you have PCI-E, but you really don't.