You shouldn't need to "update" DX11 for Win 7, only for Vista. There have been no updates to DX since the June 2010 SDK.
This is lastest DX runtime you should be using.
http://www.microsoft.com/en-us/downl...s.aspx?id=8109
I'd also get Steam to Validate the CloD files too.
And as Stirwenn stated, make sure you've updated your graphic card drivers. If you've got an AMD card get the 12.6Beta drivers, the 12.5 drivers caused BSODs.