Just in case lets list some standard things.
1) Make sure you have the latest patch for the game.
2) Rename the AVI folder to AVI_back
3) Set the color depth to 16BIT
4) Turn off everything in the background like E-mail/ICQ/Messenger, Virus protection and everything in the system tray beside the clock.
5) Run a Virus check and a Adware/Spybot checker
6) Update your Windows, update your Video card drivers and audio card drivers. Then repeat steps 1-4.
Please run a DirectX Diagnostic:
Start menu –Run - then type Dxdiag, then select each tab across the top of the window, and run the tests on each tab. Then select “Save All Information” button in the lower right of the window. This will bring up a dialog box to allow you to select a name for the file where the information will be saved. You can save it right on the desktop as this will make it easy to find. Then copy the contents of that file from the start to the heading that says “DirectX components” then paste what you have copied into a post in this thread.