BadImageFormatException during .Net assembly load issue

When I try to access a page (default.aspx) in a web site in IIS 7.0 (developed using VSTS 2010 + .Net 4.0 on Windows Server 2008), I met with the following error message. Any ideas what is wrong? What means BadImageFormatException?

BTW: I am running Windows Server 2008 64 bit (not R2) and not activated Windows Server 2008 yet, could it be a root cause?

[BadImageFormatException: Could not load file or assembly 'test.foo' or one of its dependencies. Try to load bad formatted program. ]
   System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +0
   System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection, Boolean suppressSecurityChecks) +567
   System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +192
   System.Reflection.Assembly.Load(String assemblyString) +35
   System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +118

[ConfigurationErrorsException: Could not load file or assembly 'test.foo' or one of its dependencies.  Try to load bad formatted program. ]
   System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +11392147
   System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() +484
   System.Web.Configuration.AssemblyInfo.get_AssemblyInternal() +127
   System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +334
   System.Web.Compilation.BuildManager.CallPreStartInitMethods() +280
   System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicatio