Windows 7 Snipping Tool

If you haven't discovered the Snipping Tool in Windows 7, it's worth checking out.  The Snipping Tool allows you to capture any portion of your screen, so you can then insert the image into a document  (for you Mac users, you can accomplish the same thing using Command + Shift + 4).  I have used the tool here to give you the idea* (and to show you where to find it):

You can add annotations-- such as the red arrow above.
The image is automatically copied into your clipboard, so you can paste it where needed.  You can also save the image for future use.

Teachers and students can use this tool to incorporate screen captures into text documents and presentations.

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*note that capturing an open menu (as in my example) is a little trickier.  Here are the steps (from the Microsoft site):

1. Open Snipping Tool by clicking the Start button , clicking All Programs, clicking Accessories, and then clicking Snipping Tool.
2. Press ESC, and then open the menu that you want to capture.
3. Press CTRL+PRINT SCREEN.
4. Select the type of snip you want, and then capture the menu.