Why cant I download DirectX?

I am trying to update my Direct X so I can play the Rift trial this weekend. However, when I try to update it, it tells me an internal system error has occured and tells me to refer to DirectX.log. I accessed this file via "Run" and the top line says: "DXSetup: No command line switch". I am not sure what a command line switch is as I am not the best person in the world at computers. I have tried to download DirectX via Rift and the official microsoft DirectX website, getting the same result from both. Please help me resolve this. Thanks.