Well the reason I brought it up is because you want to make sure monitor #1 is the monitor you normally use, not a projector or some other.
You might look over this
article from Microsoft and maybe try the Mr Fix-it app. I suspect, if it is not what monitor is set as default, that it is the video card & driver.
The other possibility is something odd with the EDID reported from the monitor. But since it works fine on one computer and not the other I doubt this is the issue but you could rule it out entirely by trying a different brand monitor.