I created a new Web Application for my new Project and then created a Site Collection into it. But when I navigated to this new site url, it displayed configureation error :

Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: The entry 'WSS_Medium' has already been added.

Source Error:

Line 102: <securityPolicy>
Line 103: <trustLevel name="WSS_Medium" policyFile="D:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\config\wss_mediumtrust.config" />
Line 104: <trustLevel name="WSS_Medium" policyFile="" />

So, I modified the web.config with following tags :

<trustLevel name="WSS_Medium" policyFile="D:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\config\wss_mediumtrust.config" />

<trustLevel name="WSS_Minimal" policyFile="D:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\config\wss_minimaltrust.config" />

Now, I am able to atleast see the default page of the site but all webparts on it shows the error :

Web Part Error: This page has encountered a critical error. Contact your system administrator if this problem persists.

My all other existing Web Applications and sites are working perfectly, they do not have any error into it. And even i am able to create new site collection into it without any error... This problem is with all new Web Applications...

If anyone has idea about what is the problem and what can be the solution then please let me know...