Following on from my earlier post describing my problems
installing Visual Studio 2012 RC. It turns out that some errant installer had set two values for my
ComSpec should only ever point to one copy of
cmd.exe - it’s not a set of options, just a way to find
when you need it.
Removing the second copy fixed things, and now VS2012RC installs just fine.
Of course, the next step is to try and identify why it got broken in the first place!