In the Task Manager, click the Applications tab, then select the pop-up window from the list and click the End Task button.
Press simultaneously the CTRL, ALT, and DEL keys, and, from the resulting window, click the Task Manager button. If that doesn't work, you need to use the Windows Task Manager to close the pop-up. Right click on the button and select Close. Normally, the button and the pop-up will have the same title. To safely close a pop-up window, locate the button in your Taskbar that corresponds to the pop-up. Even buttons labeled 'Cancel' or 'Close' or a red-X button in the upper-right corner are dangerous to click doing so could trigger an infection-the opposite of what you'd expect. Specifically, you should never click anywhere in a pop-up window.
To protect yourself from serious malware infection, you must be extraordinarily careful about how you close a pop-up window. Even if your browser is set to block pop-ups you're still going to get some because no pop-up blocker is perfect. Most are merely annoying but a significant number- such as the 'Antivirus 2009' pop-up described below-are designed to infect your computer with spyware that makes your computer unusable. When you browse the web with a Windows computer you're sometimes presented with pop-up windows-and 'pop-under' windows that you don't notice until you close your main browser window.
