Require that bash.exe is on the path, along with the rest of Cygwin.

Tools/buildbot/buildmsi.bat

@@ -3,7 +3,7 @@ cmd /c Tools\buildbot\external.bat
 call "%VS71COMNTOOLS%vsvars32.bat"
 cmd /q/c Tools\buildbot\kill_python.bat
 devenv.com /useenv /build Release PCbuild\pcbuild.sln
-c:\cygwin\bin\bash.exe -c 'cd Doc;make PYTHON=python2.5 update htmlhelp'
+bash.exe -c 'cd Doc;make PYTHON=python2.5 update htmlhelp'
 "%ProgramFiles%\HTML Help Workshop\hhc.exe Doc\build\htmlhelp\pydoc.hhp
 cd Tools\msi
 del *.msi