Check graphics settings. You will need to set everything to lowest first.
Tick off antialiasing. Tick off reflections. Tick off vertical sync. Tick on low fx.
Set shadows to off. Set textures to low. Set physics to low. Set clutter to off.
Set max foreground frames to 100, and ticked on.
For background frames, i normally set it low, but it shouldn't matter since that is when you are alt-tabbed in windows.
Now go in game and if its smooth you can start turning some of the settings back up.