The VS 10 msbuild tool uses a single command shell to invoke all the
custom command scripts in a project.  Isolate the environment and
working directory of custom commands using setlocal/endlocal.  The
form of each command is

  set errlev=
  setlocal
  cd c:\work\dir
  if %errorlevel% neq 0 goto :cmEnd
  c:
  if %errorlevel% neq 0 goto :cmEnd
  command1 ...
  if %errorlevel% neq 0 goto :cmEnd
  ...
  commandN ...
  if %errorlevel% neq 0 goto :cmEnd
  :cmEnd
  endlocal & set errlev=%errorlevel%
  if %errlev% neq 0 goto :VCEnd

so that all changes to the environment and working directory are
isolated within the script and the return code is preserved.