Just like many of you I updated my workstation with windows 8.1 Update 1. I also installed vs2013 with the RC update to begin developing Universal Windows Apps. However when I ran the app on my W8.1 machine with the update I got the exception shown below:
Class not registered (Exception from HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG)) at Windows.UI.Xaml.Application.Start(ApplicationInitializationCallback callback) at App1.Program.Main(String args)
This project was a default as it could be. It was created using File -> New Project -> Universal App template. No custom code was added to it. After trying every fix on the web, looking in the eventlog and determining that no error was added there I resorted to ProcMon. Then I noticed Avast was very busy when I ran my app.
The reason that the exception was thrown was that Avast Antivirus was scanning my app that I ran from visual studio. Slowing the startup of my app until I got the exception showed earlier. After disabling the filescanner for my visual studio project directory my apps launched perfectly.