To take a snapshot of a window, put the mouse pointer inside the window you want, press Alt - PrtScrn, and an image of the window is put in the Clipboard. Now go to FrontPage, Word, PowerPoint, Paint, or whatever and Edit, Paste.
To take a snapshot of the entire screen, press PrtScrn. An image of the screen is put in the Clipboard.
Sometimes you want just part of the screen or window. I find that it is easiest to paste the image into Paint, then select the portion that I want and copy and paste.
If you need to save the image as a file without pasting it into another program, the simplest way is probably to open Paint, size the current image to be small (Image, Attributes, enter small values for height and width, like 20 and 20), then paste the image into Paint and do File, Save. When you open a New window with Paint, it tends to revert to the most recently set image size, but when you paste in something bigger than that, it expands as necessary. This way you don't need to resize your image.
Screen images should be saved as png files, which will stay crisp and clear, rather than as jpg files, which is an image format.