I noticed when installing this: http://sourceforge.net/projects/opencvlibrary/files/opencv-win/2.1/OpenCV-2.1.0-win32-vs2008.exe/download
("add to system path for all users")
that it *deleted* my system PATH environment variable, and replaced it with (only)
which is obviously bad. I realize this is probably caused by some other problem but just noting it since it surprised me.