Well, it sounds like you lack a video card past an onboard?
Onboard cards truly, absolutely suck. This game's code isnt even all that optimized (In my opinion) so it makes getting proper performance a task only accomplished through really tweaking the game.
For starters, turn down all the graphical settings - especially things like shadows, antialiasing, anisotropic filters, etc. Also try turning down your resolution and closing all background programs that could be taking up resources from your machine.
I recommend you go to your local computer store, preferably a custom store (to save money - I typically save hundreds on computer parts a year when I buy from custom stores as opposed to retail) and buy yourself a video card. Ask the people there for a decent one that will work for gaming (make sure to state the game) and is within your price range. I've ran on two video cards less than $200 for the past 5 years without very many issues other than not being able to max out every setting, and I can play any game on the market just fine right now.