The "ValidateXaml" task failed unexpectedly.

If you are reading this, that means you must have ran into this exception

Error The "ValidateXaml" task failed unexpectedly.
System.IO.FileNotFoundException: Could not load file or assembly 'System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e' or one of its dependencies. The system cannot find the file specified.
File name: 'System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e'
at System.ModuleHandle.ResolveType(Int32 typeToken, RuntimeTypeHandle* typeInstArgs, Int32 typeInstCount, RuntimeTypeHandle* methodInstArgs, Int32 methodInstCount)
at System.ModuleHandle.ResolveTypeHandle(Int32 typeToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext)
at System.Reflection.Module.ResolveType(Int32 metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments)
at System.Reflection.CustomAttribute.FilterCustomAttributeRecord(CustomAttributeRecord caRecord, MetadataImport scope, Assembly& lastAptcaOkAssembly, Module decoratedModule, MetadataToken decoratedToken, RuntimeType attributeFilterType, Boolean mustBeInheritable, Object[] attributes, IList derivedAttributes, RuntimeType& attributeType, RuntimeMethodHandle& ctor, Boolean& ctorHasParameters, Boolean& isVarArg)
at System.Reflection.CustomAttribute.GetCustomAttributes(Module decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes)
at System.Reflection.CustomAttribute.GetCustomAttributes(Assembly assembly, RuntimeType caType)
at System.Reflection.Assembly.GetCustomAttributes(Type attributeType, Boolean inherit)
at System.Attribute.GetCustomAttributes(Assembly element, Type attributeType, Boolean inherit)
at MS.Internal.Xaml.MS.Impl.XamlSystemBinding.AddPairsFromAssembly(XamlSchemaContext schemaContext, Assembly asm, String uri, List`1 pairs)
at MS.Internal.Xaml.MS.Impl.XamlSystemBinding.SearchForXamlNamespace(XamlSchemaContext schemaContext, String uriString, IEnumerable`1 referenceAssemblies)
at MS.Internal.Xaml.XamlSchemaContext.GetXamlNamespace(String xmlns, Assembly localAssembly)
at MS.Internal.Xaml.XamlSchemaContext.GetXamlNamespace(String xmlns)
at MS.MarkupCompiler.ValidationPass.ValidateXaml(String fileName, Assembly[] assemblies, Assembly callingAssembly, TaskLoggingHelper log, Boolean shouldThrow)
at Microsoft.Silverlight.Build.Tasks.ValidateXaml.XamlValidator.Execute()
at Microsoft.Silverlight.Build.Tasks.ValidateXaml.XamlValidator.Execute()
at Microsoft.Silverlight.Build.Tasks.ValidateXaml.Execute()
at Microsoft.Build.BuildEngine.TaskEngine.ExecuteInstantiatedTask(EngineProxy engineProxy, ItemBucket bucket, TaskExecutionMode howToExecuteTask, ITask task, Boolean& taskResult)


Here's the fix
1. Uninstall all silverlight tools from your box. (that includes version 2.0, 3.0, visual studio tools for 2.0, visual studio tools for 3.0)
2. Install visual studio 2008 sp1 once more.
3. Reboot (yes they dont tell you, but that will save you lots of time).
4. Install the silverlight tools
5. Install in GAC System.Core which is located at (C:\Program Files\Microsoft SDKs\Silverlight\v2.0\Reference Assemblies)