If at a Windows NT or Windows 2000 computer, follow these steps.
Press and hold down the Ctrl, Alt and Delete keys
on your keyboard.
You will get a screen with the following options:

Under the Applications Tab, you will see a list of all the programs running on your workstation. To the right of each task, Task Manager displays the status of each task. Look for a task with a status of "Not Responding" -- this task is the one that has your machine locked up. Any task with the Status of "Running" is working fine.
Select the application that is "Not Responding" then click
the button labeled "End Task". You may receive a warning
box asking you to confirm that you really want to end this task. Click "End
Now" to close the application.
WARNING: YOU WILL LOSE ANY UNSAVED WORK IN THIS APPLICATION!