Normally it's necessary to click a window to make it active, but it is possible to avoid this by using one of Vista's Ease of Access features. Click Start, open the Control Panel and then open the 'Ease of Access Center'. Click the link labelled 'Make the mouse easier to use' and then select the option labelled 'Activate a window by hovering over it with the mouse'. Windows will now activate when the mouse passes over them. If you don't find this helpful, it's easy to deselect the option from the same window.

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