window phone 7 error

I have a project that worked fine but today i get this error... even when I creating new project i get same message... I can't understand what is the problem...

Error   1   The "CompileXaml" task failed unexpectedly.
System.IO.FileLoadException: Could not load file or assembly 'System.Windows, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' or one of its dependencies. Strong name validation failed. (Exception from HRESULT: 0x8013141A)
File name: 'System.Windows, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' ---> System.Security.SecurityException: Strong name validation failed. (Exception from HRESULT: 0x8013141A)

The Zone of the assembly that failed was:
MyComputer
   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.InternalLoadFrom(String assemblyFile, Evidence securityEvidence, Byte[] hashValue, AssemblyHashAlgorithm hashAlgorithm, Boolean forIntrospection, Boolean suppressSecurityChecks, StackCrawlMark& stackMark)
   at System.Reflection.Assembly.LoadFrom(String assemblyFile)
   at Microsoft.Silverlight.Build.Tasks.CompileXaml.LoadSilverlightAssemblies(String[] frameworkPaths)
   at Microsoft.Silverlight.Build.Tasks.CompileXaml.GenerateCode(ITaskItem item, Boolean isApplication)
   at Microsoft.Silverlight.Build.Tasks.CompileXaml.Execute()
   at Microsoft.Build.Framework.ITask.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask, Boolean& taskResult)

Best Answer of Carl Saldanha (read comments)

Need re-installation...