THe 6600 comes in AGP as well as PCIE interfaces. So when you say you cannot install the card it could be because the card does not physically fiit into the slot. More information is needed including the MB model and the card specs also.
The Intel 845gl/gv board has onboard video and is not intended for use with a discrete graphics card. It has 3 PCI ports but not even the older AGP video slot let alone PCIe.
Edit: I am not aware of the graphics requirement for Windows 7 so don't know if any of the above will provide the graphics power needed - any others up on this?