It's hard to search and to explain it, being English not my native language, anyway I'll try:
ALT+TAB works fine on my PC, but only if I use just keyboard; I was used to use it in conjunction with mouse: I pressed alt+tab, then I clicked on the desired window.
But it does not work anymore: mouseclick does not have any effect on window selection; indeed, the mouseclick is not detected by the app-switch-popup, but by the window behind!
I am using Windows 7 but I am not administrator of my office PC.