The AsyncCTP is getting a bit long in the tooth. This post is to explain how, why, and what to do.
The AsyncCTP is an update. It updates Visual Studio 2010 with modified versions of the VS IDE compilers that colorize the new keywords. It also updates the .NET4 framework with modified versions of the command-line csc.exe and vbc.exe compilers. But the AsyncCTP was never incorporated into Microsoft's official source code for VS2010 or .NET4, and was never meant to be -- it's just an experimental CTP.
And so, when Microsoft produces official updates to either VS or .NET4, and they touch the same files, then they tend to overwrite the AsyncCTP -- or prevent it from installing in the first place.
Also .NET45, which comes with VS11 and Win8, is an in-place update on top of .NET4, and so replaces the csc.exe and vbc.exe from the AsyncCTP with updated ones. As Stephen Toub explains, the new csc.exe and vbc.exe are incompatible with AsyncCTPLibrary.dll.
NB. The AsyncCTP installer has a problem with network drives: if your My Documents is mapped to a network drive, then it won't install.
We understand async has become indispensable for many of your projects. Ultimately, though, we can't support the AsyncCTP -- it's too fragile and has too many bugs. It took us an entire release cycle to fix it, and VS11/.NET45 is the result. The only long-term supportable strategy is for people to migrate over to VS11. As soon as we've figured out the migrate story, we'll let you know.
PS. If you notice other updates which conflict with AsyncCTP, please post in the comments here, and I'll update the instructions.