You CANNOT insert an AGP card into a PCI slot as the size of boards connections are different (in most cases) and the voltage requirements are different (in all cases).
If you succeeded in inserting an AGP into a PCI slot, it would probably not work (not enough voltage).
To fix, get a motherboard with an AGP slot, or get a different device of the same type that is PCI instead of AGP.
Matt A+, MCP, MCP+I, MCSE Windows NT 4.0, MCSE Windows 2000 Early Acheiver with Security Emphasis