Have all instances of Visual Studio been closed? I would expect this behaviour if a VAX dll is still loaded so that it cannot be replaced immediately. As far as I understood the VS detection mechanism of VAX it is just an enumeration of main windows and the lookup of "Microsoft Visual Studio" in the window title. So maybe your msdev.exe or devenv.exe were still running/hanging after all IDE windows have been closed?
Well, hanging possibly, but I always shut all instances of VS before I run the VA install, because earlier versions didn't do the 'already running' check.
Maybe it was a bug in 1223 which caused shutdown hangs....?
I don't know why it hung, but when I tried to install 1225 at home, it came up with the dialog telling me to close all instances of MSDEV before proceeding, and sure enough, there was an instance in the task list but no window.
My upgrade from 1223 to 1225 went flawlessly - after I closed a text editor with a file open from a sub directory of Mirosoft Visual Studio because the editor shows the complete file name in its title it was expected to be a running Visual Studio