Used to play this game before, 2-3 years ago. I played with my old computer and also an old laptop, - everything worked fine

Now... I have a new computer (which I bought 4 months ago), and it won't let me play... CPU gets boosted up to 100% and everything lags very very very very much. What do I do wrong?

HP Pavilion G7
Windows 8
750GB hard drive
AMD A6-4400M with Radeon(tm) HD Graphics
Radeon HD7670M

Any help?