Firstly, please make sure that your PC meets the The Witcher 2: Assassins of Kings system requirements.
If the requirements are met:
1. Please make sure that all applications non essential for your PC are turned off/disabled. This includes antivirus/antimalware programs.
If that won't help please check if your PC is malware/virus free - those may sometimes cause crashes.
2. Collecting too many saves may lead to crashes. Delete the unneeded saves. Do it using Delete option in Load Game menu or straight in the folder where the saves are stored: (...)Documents\Witcher 2\gamesaves.
Steam only: Once cloud saving is activated the saves are stored in the cloud and in (...)Steam/userdata/<user>/20920/remote . The cloud contents will overwrite the folder's contents if anything is changed within the folder. This means that saves can be removed straight from the folder only if cloud saving is deactivated.
3. Decrease Texture Memory Size.
If you have less than 4GB RAM, go to Launcher > Advanced Options > lower Texture Memory Size.
4. You may also try verifying game files, as described here.
5. If any of the above won't help, reinstall your GPU drivers. To do so completely follow these steps:
Please be aware that sometimes installing older drivers might help with crashing.