It's almost certainly your graphics, as the above poster states. Your graphics hardware is usually the most important piece of hardware when gaming and the first to cause a bottleneck, and if you are using the on-chip graphics in your processor then that would be the cause.
There's an easy way to identify your hardware. Open the start menu and type 'dxdiag' (without apostrophes) into the search bar. Press enter and click either option at the prompt, then wait for a window to pop up. Your graphics hardware should be listed under the display tab.
After you know what graphics hardware you're using you should install the latest drivers for it and try again with the lowest settings and resolution that you can cope with, but keep in mind that it might be a hardware limitation that you can't overcome without buying a new card.
PS. It's a shame that you can't run it because Sonic Generations is a really good game and the port doesn't have any specific problems with performance afaik