If I'm not mistaken, Win7 Pro has Media Center included. It does in my 64bit version. Try that and I bet it will work fine.
I can see the same sync issue only on HD channels when using the WinTV Windows app. Changing the video renderer from EVR to VMR9 doesn't help. Running 2.3d.
I can make WinTV work correctly under the following conditions, and that is to set Power Option to High Performance power plan. In Balanced power plan, I can see the P-state fluctuate and the video render can't keep up.
All video and sound card drivers are up to date.
Hope this helps. Mr. X, I jumped in here because I had been meaning to start a thread specifically on the out of sync issue with WinTV program.
BTW, Extend streaming HD to an iPhone 4 does not have this problem.
Extend streaming HD to a PC browser has issues where the stream feed can't keep up, and eventually the audioless slowmotion phenomenon happens. This happens over a 100Mb network connections to a remote client PC, as well as the native PC that is hosting the Extend server itself.