Random CTD can be hard to track down, what were you doing when they happened?
Also let's try 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) Update your Windows, update your Video card drivers and audio card drivers. Then repeat steps 1-3.