PC game I just got Lags when I play it help?

So, I just got Overlord for the PC. Now the cut scenes are fine no lag at all. But when gameplay begins it starts to lag. I have a Advent Roma 3000 Laptop. 4096MB DDR2 RAM. Intergrated Intel Mobile 4 Series graphics card. And a 500 GB Hardrive. Can I do anything to ensure my game will not lag? Or will it be better to use the PC I have downstairs? Thanks.