Well, checking twice, I'm not sure it's even related because cc1.exe is outside of the PATH's anyway. Also, when the dev-tools page says only that one path is required, I doubt it makes a difference... however, that's how it works here .
A mind is like a parachute. It doesn't work unless it's open. [Frank Zappa]
Weren't you already using the "RC version" of the new tools successfully? When those work, the actual new tools should work as well.
No, after installing the unofficial GCC 4.6 and Qt 4.8.1 to compile QCHDman I tried to compile MESS too and I got a cc1.exe: internal compiler error. I think I told you or Micko this already in the past.
As Anna said, the last official tool that works for me with WinXP SP3 32-bit, is mingw-mame-w32-20110519.exe; GCC 4.4.7.
The only problems I have with the current source plus messui 0.146u5 source from http://messui.the-chronicles.org/ is some #pragma GCC diagnostic errors for unknown objects. Comment out the pragmas and r17490 built to completion.