Could you please post your system specs for us, so we can help you a bit more by understanding exactly what the problem is.
Include:
CPU speed (500MHz, 1.5GHz, 2GHz? etc)
RAM amount (256mb, 512mb, 1gb? etc)
Graphics card (ATI Radeon, GeForce? etc)
Also please tell us if all relevant drivers are up to date on your computer. This includes drivers for your graphics card, sound card, BIOS, DirectX.
Hope we can help you.
