If you open the software for the camera, you can see your video and take a snapshot. There will be a button in the software to take a snapshot. It's software from the manufacturer, not form windows. Kind of like the camera drivers.
Some external webcams also have a button on them for taking a snapshot. But the usual way is to use the cameras software.
Using Windows 7 go to Control Panel, Hardware and Sound, Devices and Printers, then double click your webcam and it will open its software. You can tweak all the settings there and view your webcam and take photos or video.