Two different implementations in one computer

NeumannTuring

Hello,
I used opengl for one course a year ago(I do not remember which
implementation I downloaded and where it came from). Anyway everything
was fine at that time except it failed to render big pictures(probably
because my computer does not have good graphic card). Now, I need to
use xming which said it requires Mesa Libraries-a software
implementation of OpenGL. But after I installed Mesa , xming does not
work---display failure. Worse, I find that the programs I wrote before
have uglier output than before. Is it because Mesa supercedes the
previous implementation in my computer? How can I force my opengl
program to use the previous implementation instead of Mesa ? Besides,
do you have any ideas why xming fails?
Thank you.