Old DLL file keeps being used

Who knows where Visual Studio is getting this dll from - it's months out of date!

The Modules Window is your friend...

It'll tell you exactly where that file is coming from. You can even use it with arbitrary processes if you attach the debugger.

It hides it in the GAC. There it may reside indefinitely. Using a more recent version may indeed solve the problem, but there is an outstanding bug in Visual Studio that has to do with choosing the correct version of DLL files. (If DLL Hell wasn't bad enough, the Visual Studio team is making it worse!)

Finding it in the GAC is tricky, and I cannot advise you on how to do that, but once the old version is deleted from there, it will not be found again. Sometimes, even though you are pointing the compiler at a newer version (by date), it will use the older version, because it has the same version level (by version). That is its bug.

I too would guess that they're hiding in the GAC. You can look in 'C:\Windows\assembly' to see all the dlls and unregister yours from there.