Could not load file or assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.

When trying to browse my WCF service using .Net Framework 4.0 and hosted in IIS 6.0 I was getting this error in the event viewer.

Event Type:         Error

Event Source:     ASP.NET 4.0.30319.0

Event Category:  None

Event ID:            1325

Description:

Failed to initialize the AppDomain:/LM/W3SVC/1381374284/Root/RetrieveDataService

Exception: System.IO.FileLoadException

Message: Could not load file or assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. Access is denied.

StackTrace:    at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)

   at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)

   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection, Boolean suppressSecurityChecks)

   at System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection)

   at System.Activator.CreateInstance(String assemblyName, String typeName, Boolean ignoreCase, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, Evidence securityInfo, StackCrawlMark& stackMark)

   at System.Activator.CreateInstance(String assemblyName, String typeName)

   at System.AppDomain.CreateInstance(String assemblyName, String typeName)

   at System.AppDomain.InternalCreateInstanceWithNoSecurity(String assemblyName, String typeName)

   at System.AppDomain.InternalCreateInstanceWithNoSecurity(String assemblyName, String typeName)

   at System.Activator.CreateInstance(AppDomain domain, String assemblyName, String typeName)

   at System.Web.Hosting.ApplicationManager.CreateAppDomainWithHostingEnvironment(String appId, IApplicationHost appHost, HostingEnvironmentParameters hostingParameters)

   at System.Web.Hosting.ApplicationManager.CreateAppDomainWithHostingEnvironmentAndReportErrors(String appId, IApplicationHost appHost, HostingEnvironmentParameters hostingParameters)

Resolution

Because the Application Pool was using the identity NETWORK SERVICE so I had to give the IIS_WPG group (which contains NETWORK SERVICE) read/write access to my web root directory.