Designer crash by Exelle

Whenever I open up the XAML designer (by double clicking on a xaml file), I get an exception in the visual designer half of the split screen (the xml code half seems to be running ok, but without any autocomplete.)

the error I see in the designer page is:
System.Exception
Package could not be registered. (Exception from HRESULT: 0x80073CF6)
at Microsoft.Expression.HostUtility.Platform.AppContainerProcessDomainFactory.CreateDesignerProcess(String applicationPath, String clientPort, Uri hostUri, IDictionary environmentVariables, Int32& processId, Object& processData)
at Microsoft.Expression.DesignHost.Isolation.Primitives.ProcessDomainFactory.ProcessIsolationDomain..ctor(ProcessDomainFactory factory, IIsolationBoundary boundary, AppDomainSetup appDomainInfo, FrameworkName targetFramework, String identifier, String baseDirectory)
at Microsoft.Expression.DesignHost.Isolation.Primitives.ProcessDomainFactory.CreateIsolationDomain(IIsolationBoundary boundary)
at Microsoft.Expression.HostUtility.Platform.AppContainerProcessDomainFactory.CreateIsolationDomain(IIsolationBoundary boundary)
at Microsoft.Expression.DesignHost.Isolation.Primitives.IsolationBoundary.Initialize()
at Microsoft.Expression.DesignHost.Isolation.Primitives.IsolationBoundary.CreateInstance[T](Type type)
at Microsoft.Expression.DesignHost.Isolation.IsolatedExportProvider.Initialize()
at Microsoft.VisualStudio.ExpressionHost.Services.VSIsolationService.CreateExportProvider(IIsolationTarget isolationTarget, ICatalogFactory catalogFactory, IExportFilter filter)
at Microsoft.Expression.DesignHost.Isolation.IsolationService.CreateLease(IIsolationTarget isolationTarget)
at Microsoft.Expression.DesignHost.IsolatedDesignerService.CreateLease(IIsolationTarget isolationTarget, CancellationToken cancelToken, DesignerServiceEntry& entry)
at Microsoft.Expression.DesignHost.IsolatedDesignerService.IsolatedDesignerView.CreateDesignerViewInfo(CancellationToken cancelToken)
at Microsoft.Expression.DesignHost.Isolation.IsolatedTaskScheduler.InvokeWithCulture[T](CultureInfo culture, Func`2 func, CancellationToken cancelToken)
at Microsoft.Expression.DesignHost.Isolation.IsolatedTaskScheduler.<>c__DisplayClassa`1.<StartTask>b__6()
at System.Threading.Tasks.Task`1.InnerInvoke()
at System.Threading.Tasks.Task.Execute()

this has happened from the very first time I installed VS2012. I tried repairing the instalation (from the install/uninstall menu), and then I actually uninstalled and reinstalled Visual Studio to no success.