And you've kept all of your drivers up to date? The truth is that software is becoming more and more demanding and Nvidia has dropped the 8400GS as their baseline offering. I believe the GT 210 is now their lowest model.
Without spending any money, the most you can do is;
- Clean it out
- Update drivers
- Set all Windows visual options to low presets
- Adjust power settings to high performance
- Use a solid color desktop background (no image)