Symptoms

When open the IIS Manager 7.0, then select the server In the feature view right click on the .Net Trust Level open feature the drop down option is empty when we click on the drop down we get the error as follows

netlevel

Cause:

<system.web>

<securityPolicy>

<trustLevel name="Full" policyFile="internal"/>

<trustLevel name="High" policyFile="web_hightrust.config"/>

<trustLevel name="Medium" policyFile="web_mediumtrust.config"/>

<trustLevel name="Low" policyFile="web_lowtrust.config"/>

<trustLevel name="Minimal" policyFile="web_minimaltrust.config"/>

</securityPolicy>

</system.web>

The above mentioned information from the Web Config File from the respective Asp.net Framework file

Resolution :

Verify the Different .Net framework version used by IIS

In the case consider the .Net framework Version to be 2.0.5077

For 32 bit IIS 7.0 or if 32bit mode is enabled on IIS 7.0

%Windir%/Microsoft .Net/Framework/v2.0.5077/Config

For 64Bit IIS 7.0

%Windir%/Microsoft .Net/Framework64/v2.0.5077/Config

Right Click and edit the Web Config File

An verify that the above mention value

<system.web>

<securityPolicy>

<trustLevel name="Full" policyFile="internal"/>

<trustLevel name="High" policyFile="web_hightrust.config"/>

<trustLevel name="Medium" policyFile="web_mediumtrust.config"/>

<trustLevel name="Low" policyFile="web_lowtrust.config"/>

<trustLevel name="Minimal" policyFile="web_minimaltrust.config"/>

</securityPolicy>

</system.web>

Is the above mentioned value missing ??

If yes replace the Web Config  file from backup

Reference

http://msdn.microsoft.com/en-us/library/wyts434y.aspx

** in my case it was installing the Symantec Endpoint Protection 11 MR 4 installed on Windows 2008 Web server caused the concern