Want to run WinFX Sept. CTP with VS 05 RTM?

Well you can’t. As you might be aware that the .NET runtime are different between Visual Studio 2005 RTM and WinFX Sept CTP build, and the only way you can get this to work together is fall back on Beta 2 of Visual Studio 2005 - which is the only supported  version as of now. On the other hand in about a week or so from now, a new WinFX build is supposed to be publicly released, which can coexist and keeps each other happy. If you can wait for a week I could recommend the latter.