If you don't wish to install an additional program to capture screen shots, you can easily use the PrintScreen function in Windows:
To capture the whole screen, simply press the PrintScrn button (just right of the F12 key)
This will create an image of everything on the entire screen. It then copies that image to the Windows clipboard.
Next, Go to the Start Menu/Run and type mspaint [press enter]
In mspaint, select Edit and then paste. (you can also use <ctrl + v>) You should see the image of your screen in the main window. At this point, you could edit the image, before saving.
Select the File menu, then 'Save As'
Give the file a name and from the 'Save as type', choose PNG
You now have a screen capture.
To select only the active window using this method, press the <Alt> key with PrintScrn.