I think you most likely need a video card upgrade. (I believe that your GT 240 is below the minimum requirements).
Though it's not a "top of the line" card, a nVidia 240 GT card is fairly decent, it should be fine and is a better and newer model than the Minimum-specs listed for nVidia of "Geforce 8800GT".
http://en.wikipedia.org/wiki/Comparison_of_Nvidia_graphics_processing_units
I have a GT 220 card in my old computer, and it seems to run SSE well enough for me to enjoy.
Aad the Pirate: Looking over your computer's specs, I think it passes all of SSE minimum requirements. It looks like you have DirectX and graphic drivers up-to-date.
Here's a post I made over on the Steam-forums about SSE and some general suggestions users might try to help get it up and running and may also help improve performance:
http://forums.steampowered.com/forums/showpost.php?p=19966512&postcount=3
You might want to also try some different SSE's various graphics settings, fullscreen vs windowed mode, go to nVidia's Control Panel form your graphics-card and add a profile for SSE (I set mine for when QuestViewer is running) and try some different settings there as well, such as trying with or without VSync enabled, select Single-Monitor Display, etc... Hopefully you'll be able to find a good balance of quality and performance for SSE running on your computer. Good luck.