On the problem machine, go to C:\Program Files (x86)\LyX20\Resources\fonts (assuming LyX is installed in the default location). There should be ten TrueType font files (.ttf), along with a couple of text files. Those fonts are used by LyX to display math on the screen, so it is possible that one of them is not properly installed or is broken. (Broken seems unlikely if reverting to an earlier version of LyX did not cure the problem.)
I don't have a Win 7 machine handy at the moment, but there should be a font applet in the control panel. Try reinstalling the ten font files and see if that cures the problem. You probably don't need to reinstall all of them, but offhand I don't know which one would be responsible for \leq. Paul
