Here's a check list I got from the site pertaining to that very issue, worth a try to check.
1. Check to see that the CPU, RAM, VGA and other primary and accessory components are properly installed in their respective slots. Secure them to the mainboard, and for cases when more than one hardware is installed, use only the minimum hardware to check if the game is running correctly.
2. Deactivate all overclockings. If the CPU or VGA has been overclocked, unstable conditions may occur due to overheating. If a down situation continues, reduce the core and memory clock to prevent overheating. First, try downclocking the CPU and VGA installed on the system. However, please be aware that this may cause significant damages to the system. For overclocking and downclocking a CPU or VGA, consult the Hardware Forum or the hardware manufacturer.
3. Reduce the texture to "low" and turn off reflection effects and other graphic options or minimize them.
4. Enter "dxdiag" in the Start>Execute window and turn off acceleration options for Direct Draw, Direct 3D, and AGP hardware from the "Display" tab.
5. Via board users should install the latest 4in1 patch or consult the H/W forum to minimize problems from occurring.
6. Check to see if the CPU, VGA, and power supply cooling fans are operating correctly. They may appear to be working properly, but there are a significant number of cases where the cooling does not correctly occur. Great results may be gained simply by cleaning the dust from the cooling fan.
7. Check to see if there is any hardware installed that the system's power supply cannot handle. If using many accessory components together, such as the CD ROM, DVD ROM, CD Writer, additional hard disk, or expansion cards, we recommend that you remove unneeded hardware or upgrade the power supply to 300W or more.
8. When updating the nVidia driver, if deletion of the old version has failed, this may cause a critical error in the system. Use the following program to eliminate the older version of the driver and update the new driver.
http://www.guru3d.com/detonator-destroyer/
9. If there is a Q-Fan option in the system BIOS, set this to "Disable."
10. If you are a WindowsXP user, try disabling the "Use Write Combination" option from the Display Information>Problem Solving tab