We are using a Windows server with multiple people logging in simultaneously. From that windows server multiple people can log into the same unix server. On the windows console each user starts XMing to run X-applications on the unix server.
We saw that the X-client started by a user appeared inside the windows console of the other user. So the user who started for example xterm or xedit did not see the xterm or xedit window. Instead these windows popped up on the windows console of another user logged in at the same time on the same windows machine.
This error can be prevented by configuring XMing using XLaunch and our putty ssh-session.
Within XLaunch you have to select "Multiple windows". In the same screen set the display number to a unique number, say 59. Each windows user has to use his own display number that is different from any of the display numbers of other users. Then choose "Start no client". Finally save the configuration and start XMing by double clicking on the saved configuration file.
Within putty you have to configure your ssh session to automatically set the unix DISPLAY variable to the display number you have chosen in the previous step. You do that by navigating to "Connection", "SSH", and then "X11". Chose "localhost:59" if you have set "59" as your display number in the first step.
Save your putty session and use it to log in to the unix machine. You can now start xterm or xedit. These xclients will appear on your own windows console. Surprisingly these XLaunch/putty settings do not affect the value of the DISPLAY environment variable in your unix session.