There is some kind of malware installed on a client's computer. It effectively prevents the administrator account from being used.
It pops up a fullscreen window which you can't get around: Alt+Tab does show the list of windows, but switching doesn't work. You can start the task manager, but it loses focus. However, ordinary user accounts seem not to be affected. I tried to find out the malicous process using processexplorer while simultaneously logged into another user account--but couldn't find any suspicous processes.
Is it possible to find out which process has opended that window while logged into another user account? The drag&drop inspector, which comes with processexplorer of course doesn't work, because you can't move the mouse to another session :-)