Symptom:

AccelSee was installed on the same server (with SharePoint)

HTTP Error 500  was shown when trying to open SharePoint

There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined 

Solution

1. Open your IIS console
2. Change SharePoint - 80, SharePoint Central Administration, SharePoint Web Services Root into .NET Framework 2.0
3. Restart All application pools
4. Restart SharePoint Web application.
5. Done