I have a Dell Optiplex 980 machine. Two monitors (17 in. and 22 in.) are attached to it. The smaller one was attached from begning, later on I added the larger one. The smaller one on left as primary monitor and larger one on right as secondary monitor (with desktop extended on both monitors).
Yesterday I re-organized my workspace and I have to move larger one on the left. I set the large monitor as primary in Display Properties dialog. It is fine with only glitch that I have to take my mouse cursor on the smaller monitor from left edge of the larger (primary) monitor.
I tried by inter-changing the connecting cables but Windows 7 still refuses to believe that larger monitor is on the left and smaller one on the right side.