A computer, operating system, software program or driver may stop responding or cause other programs to stop responding because of several possible reasons, such as a conflict of software or hardware resources between two programs, lack of system resources, or a bug in the software or drivers.
To recover from a program that has stopped responding, a user can generally press the CTRL + ALT + DEL keys on his or her keyboards to open the "Close Program" window or open the "Task Manager" window and click the "End Task" button to terminate the program. If another window opens, click the "End Task" button again to confirm stopping that program.
It is important to realize that when a program stops responding any work that has not been saved will more than likely be lost when end tasking a program. Unfortunately, there is no alternative.
Source: Miniden Forum