Lightshot is a program that you can install for free in Windows (or Mac).
When installed, just press the Print Screen button and then select the area on the screen that you want captured.
When an area has been selected, it is possible to add text, arrows, boxes or freehand forms. When done, it's possible to copy the result to clipboard, save it to disk, print it or search for similar images on Google.
A really handy tool!