Windows 7 has a little gem built right in called Snipping Tool. You can launch it from the Start button, All Programs, Accessories. For years I always needed a tool to capture the screen to an image file. You can save the image as PNG, JPG or GIF. If you use this often I recommend pinning it to the task bar (Windows 7 only feature). Pinning an application is easy, just right click the program from the start menu and select "Pin to Taskbar".
The concept is easy, when you launch the program your screen turns a hazy white and the Snipping Tool window pops up. Just highlight the area you want captured by dragging the mouse from one corner to the other corner while holding down the left mouse button. Now comes the cool part, it has the ability to draw on your screen capture. You could circle something you need to draw attention to. You have the choice of several colors and pen thicknesses. You also have a yellow highlighter. There is even an eraser to undo your markups.