Maybe your specific 8400GS isn't compatible?
GeForce 8300 and 8400 Series
NVIDIA GeForce 8400 GS
In the summer of 2007 NVIDIA released the entry-level GeForce 8300GS and 8400GS graphics cards, based on the G86 core. The GeForce 8300 was only available in the OEM market, and as the GeForce 8300 mGPU motherboard GPU. These graphics cards were not intended for intense 3D applications such as fast, high-resolution video games. They were originally designed to replace the 7200 and 7300 models, but could not due to their poor game performance. It was able to play modern games at playable framerates at low settings and low resolutions making it popular among casual gamers and HTPC (Media Center) builders without a PCI Express or AGP motherboard.
At the end of 2007 NVIDIA released a new GeForce 8400 GS based on the G98 (D8M) chip.[8] It is quite different from the G86 used for the "first" 8400 GS, as the G98 features VC-1 and MPEG2 video decoding completely in hardware, lower power consumption, lowered 3D-performance and a smaller fabrication process. The G98 also features dual-link DVI support and PCI Express 2.0. G86 and G98 cards were both sold as "8400 GS", the difference showing only in the technical specifications. Sometimes this card is called "GeForce 8400 GS Rev. 2".
During mid-2009 NVidia released another revision of the GeForce 8400 GS based on the GT218 chip.[9] It has a larger amount of RAM, is capable of DirectX 10.1, OpenGL 3.3 and Shader 4.1. This card is also known as "GeForce 8400 GS Rev. 3".
http://www.geforce.com/geforce-experience/system-requirements